clothing systems for outdoor activities

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Clothing systems for outdoor activitiesMatthew P. Morrisseya & René M. Rossiaa Empa – Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Protection and Physiology, Lerchenfeldstrasse 5, CH-9014 St.Gall, SwitzerlandPublished online: 19 Feb 2014.

To cite this article: Matthew P. Morrissey & René M. Rossi (2013) Clothing systems for outdoor activities,Textile Progress, 45:2-3, 145-181, DOI: 10.1080/00405167.2013.845540

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Clothing systems for outdoor activities

Matthew P. Morrissey* and Ren�e M. Rossi

Empa – Swiss Federal Laboratories for Materials Science and Technology, Laboratory forProtection and Physiology, Lerchenfeldstrasse 5, CH-9014 St. Gall, Switzerland

(Received 4 February 2013; final version received 25 July 2013)

Participation in outdoor activities can improve mental, physical and social well-being.Such activities also present significant physiological strain and risks such as hypother-mia; therefore, correct choice and usage of clothing is extremely important. The aimof this review is to critically analyse the literature regarding outdoor clothing systems,focusing on the layers comprising a typical clothing system. Additionally, alternativesystems, potential improvements and future trends are discussed.

Keywords: Outdoor activities; performance clothing systems; human safety andcomfort; product design

1. Introduction

The number of participants in recreational outdoor pursuits such as land-, winter- and

water-based activities is large and growing [1]. The growing popularity of participation in

outdoor recreation activities is perhaps due to the numerous psychological and physical

benefits associated with such activities. Outdoor recreation is a major factor in human

‘wellness’, a combination of physical, mental and social well-being. In green environ-

ments, people perceive better general health, and spending time in such places has been

correlated with lower stress levels and higher amounts of physical activity [2]. Low stress

and physical activity have been shown to reduce many common health problems such as

high blood pressure, obesity, heart attacks, cancer and mental health problems [3]. Spend-

ing time in natural places is increasingly being recognised as an important ‘preventive

medicine’ with positive health, mental, social and environmental outcomes [4,5].

However, activities such as hill walking can present a significant metabolic and ther-

moregulatory strain, as well as risks such as hypothermia: though with the proper man-

agement of appropriately selected clothing, these risks can be minimised [6,7]. With the

correct clothing the benefits of outdoor pursuits can be enjoyed regardless of adverse

weather conditions, leading to the typical Scandinavian adage ‘there is no such thing as

bad weather, only bad clothing’.

The benefits of improving protective clothing for outdoor recreation therefore include

encouraging enjoyable and safe participation at a ‘grassroots’ level, and perhaps also

increasing performance and extending the limit of human endeavour in harsh environ-

ments. Innovations and improvements in this area may also have implications in other

areas such as natural disaster management (e.g. avoiding heat stress for emergency work-

ers) military applications, emergency and rescue services, and so on.

*Corresponding author. Email: [email protected]

ISSN 0040-5167 print / ISSN 1754-2278 online

� 2014 The Textile Institute

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Textile Progress

Vol. 45, Nos. 2-3, 2013, 145–181

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Consumers are subjected to a lot of information regarding outdoor clothing but this

is often skewed by industry-bias and marketing-hype [8]. Understanding the science

behind the performance of outdoor clothing may help consumers and manufacturers

make more effective and even more ecological choices regarding clothing for outdoor

recreation, protecting the natural environment in which they choose to spend their

leisure time.

1.1. Human thermoregulation and the need for protection

Humans aim to maintain thermal homeostasis with a deep body temperature of 37�C [9].

Since cell functions are highly dependent on enzymes to catalyse metabolic reactions, the

human body can only operate within quite narrow margins or deviations of þ5�C and

�10�C from the ideal deep body temperature. Metabolic processes in the human body all

result in heat production: this ‘metabolic heat’ is a product of the basal metabolic rate

(minimum requirement to maintain vital body functions), muscular activity and the ther-

mal effect of food (digestion) [9]. Since the human body operates in such a relatively nar-

row temperature range, heat transfer to and from the environment is of great importance.

Heat transfer can occur by [10]:

� Conduction. Conductive heat transfer occurs in a solid object or stationary fluid

(such as trapped air); energy is transferred from more energetic to less energetic

particles by interactions between the particles. Conduction between two objects

relies on physical contact between these objects: in the case of clothing, this may

be between fabric layers in contact with each other or between the fabric and the

human skin.

� Convection. Convective heat transfer occurs between a fluid in motion and a bound-

ing surface with different temperatures; the transfer of energy is due to the bulk or

macroscopic motion, but also may be due to random molecular motion in the fluid.

According to the nature of the flow, convective heat transfer can be classified as

‘forced’ where the flow is caused by external means, or ‘free’, where the flow is

caused by buoyancy forces due to temperature-driven density differences in the

fluid.

� Radiation. Thermal radiation is emitted by all matter with a non-zero temperature.

Thermal radiation is transferred in the form of electromagnetic waves and does not

rely on a transfer medium as do conduction and convection, i.e. it can occur in a

vacuum and in fact is most efficient in this scenario.

� Evaporation/condensation. Heat transfer may also be due to phase changes of

water, e.g. evaporation and subsequent condensation of sweat.

The human body has various methods of thermoregulation; it exploits the various

modes of heat transfer in an attempt to maintain a stable body temperature. To conserve

heat, the vascular system constricts, cooling the skin temperature (reducing the tempera-

ture difference between the skin and the environment), maximising the insulation pro-

vided by body fat and changing the thermal conductivity of the dermis by a factor of

4–10. Muscular activity in the form of shivering produces heat, and during prolonged

exposure to cold, hormones increase the basal metabolic rate. Vascular adjustments for a

nude person with normal body fat provide effective thermoregulation between 25�C and

29�C. Intense muscular activity can sustain a deep body temperature within tolerable lim-

its at temperatures as low as �30�C [11]. To dissipate heat, the vascular system dilates;

with high levels of heat loss 15%–25% of cardiac output is directed to the skin. At high

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metabolic rates sweating is the main heat-loss mechanism of the body. Water also evapo-

rates from the respiratory tract. Water evaporating from the skin and respiratory tract

extracts 580 kcal l�1 from the body. Behavioural, in addition to physiological, adaptions

can be used to control heat transfer to and from the body without resorting to clothing,

e.g. by finding a cool shade or water, or by changing the posture or building shelter, or

using fire to preserve heat.

Ideally, the heat production of the body and heat transfer to the environments should be

equal. This concept is referred to as thermal balance and is summarised by the equation:

M �W ¼ RES þ E þ Rþ C þ K þ S;

where M is the metabolic energy production, W is the effective mechanical energy, and

RES, E, R, C and K represent heat transfer by respiration, evaporation, radiation, convec-

tion and conduction, respectively. S is the change in body heat storage. Of course, thermo-

physiological and behavioural approaches to thermoregulation are not always sufficient to

maintain thermal balance and thus clothing is required. Indeed, genetic analysis of human

body lice suggests that the ‘invention’ of clothing (beyond that fulfilling decorative and

social functions, which may have occurred earlier) may have coincided with the spread of

modern Homo sapiens from the warm climate of Africa, between 50,000 and

100,000 years ago [12].

Clothing systems, in addition to physiological and behavioural processes, also func-

tion by modifying the heat transfer between the human body and the environment. One

example of an ancient, 5000-year-old clothing system is that found on a body frozen in

the Austrian Alps [13]. This clothing consisted of an assortment of furs with fastenings to

modify ventilation and a weather-resistant grass outer layer.

1.2. Modern layering systems

The most commonly used clothing system for outdoor activities is still based on a remark-

ably similar concept to that used by the ancient man found frozen in the ice. It consists of

a base layer worn for next-to-skin comfort, mid layer primarily to provide insulation, and

shell layer to provide protection from wind and precipitation. Shell layers are usually

made from waterproof breathable fabrics (WBFs) [14]. There is a popular alternative to

the traditional layering system known as ‘soft-shell’ clothing. The aim of a soft-shell sys-

tem is to provide protection from cold, wind, rain and overheating without the need to

add or remove layers, and originally without using a waterproof breathable membrane

[15]. This is achieved by combining fabric elements from the traditional layering system,

usually knitted fleece or pile fabrics, and a tightly woven shell layer with or without a

membrane, either by lamination or sewed construction. Original soft-shell clothing sacri-

ficed absolute waterproofness for increased breathability and simplicity of construction

by omitting WBFs; these garments were designed to be worn without a base layer. The

soft-shell category of garments evolved, particularly when large manufacturers of WBFs

began making soft-shell garments, such that many modern soft shells incorporate a mem-

brane and/or coating and additionally, many users combine the soft shell with a base layer.

Considering the fibres and fabrics are of extremely similar construction, the soft-shell

category of garments really represents a design approach and attitude toward dressing for

the outdoors, and therefore the information in this review can be applied to soft-shell

garments and clothing systems.

Textile Progress 147

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2. The base layer

2.1. Wetting, wicking and thermophysiology

The vast majority of product literature and many peer-reviewed articles (e.g. [16]) state

that the primary role of the base layer is to wick water away from the skin (transplanar

wicking), and also to spread the sweat over a large surface area (in-plane wicking) for bet-

ter evaporative cooling and faster drying. The science behind wetting and wicking has

been covered in several previous reviews [17,18]. Improvement of the wetting and wick-

ing properties of textile products can be achieved by a number of techniques [19]:

� By changing the fibre cross section to increase the surface area (e.g. [20,21])

� By blending hydrophilic and hydrophobic fibres into yarns (e.g. [22])

� By combining hydrophilic and hydrophobic yarns in fabrics (usually double-layer

knitted) (e.g. [23])

� By changing the surface hydrophilicity of hydrophobic fibres (e.g. chemical or

plasma treatment) (e.g. [24])

Additionally, the dynamic stretching and relaxation of yarn filaments and yarns during

fabric deformation has been shown to increase transverse wicking via a pumping effect

[25]. Oner et al. [26] and Wardiningsih and Troynikov [27] found that the liquid transport

effectiveness or ‘overall moisture management capacity’ also decreases with increasing

fabric tightness.

However, as Rodwell [28] suggested in 1965, the value of wetting and wicking from a

thermophysiological perspective is still not clear. Whether such wetting and wicking

properties are actually desirable from a thermophysiological perspective is currently

under debate. Burton and Edholm [29] recognised that because the evaporative cooling

efficiency is decreased when water is absorbed by the fabric (as the loci of evaporation

are farther away from the skin), transplanar wicking fabrics may have a detrimental

effect (i.e. exacerbating the risk of heat stress during exercise). Kerslake [30] also identi-

fied that water evaporating from the clothing would have a lower cooling efficiency.

More recent work by Havenith et al. [31,32] has quantified the reduction in evaporative

cooling efficiency as simulated sweat is systematically moved away from the skin

through the clothing layers. For example, the evaporative cooling efficiency of wet

underwear under personal protective equipment (PPE) is around 0.65, whereas that of

wet skin under PPE is close to 1. When the outer layer is wet, the evaporative cooling

efficiency is reduced further to between 0.2 and 0.4. Wang et al. [33] studied this prob-

lem with tight fitting one-layer sportswear and found that due to this effect, thinner fab-

rics are more suitable in terms of maintaining a high evaporative cooling effect. This

concept has also appeared in a popular online backpacking magazine in an article entitled

‘Just say no to wicking’, discussing the pros and cons of conventional wicking base

layers and ‘fishnet’ base layers that allow sweat to evaporate from the skin surface [34].

The concern regarding the evaporative cooling efficiency has also been used in recent

product marketing, with the manufacturer claiming that their base-layer product offers

the correct ‘balance’ between allowing optimal evaporative cooling of the skin and

removing excessive liquid from the skin, but this statement is not supported by any

empirical evidence [35].

Despite this potential thermophysiological drawback, wicking base layers may

improve comfort in terms of tactile comfort. Since human skin features no ‘sensor’ for

moisture or skin wetness sensations (but skin wetness is positively correlated with


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discomfort), moisture is detected by temperature and tactile sensors; therefore, effective

transplanar wicking, even though it may reduce the evaporative cooling efficiency, may

improve comfort sensation by reducing the strong tactile sensation caused by the ‘sticky’

wet fabric clinging to the skin [36]. Similarly, the sensation of water droplets rolling

down the skin surface will also be reduced or eliminated, and changes in temperature

caused by wet skin will be attenuated.

2.2. Drying time and water absorbed

The amount of sweat or water a base layer absorbs and the time it takes to dry is another

important issue. Commonly, advertisements, popular press and even scientific publica-

tions proliferate the idea that fabrics constructed from synthetic materials absorb very lit-

tle sweat or moisture and therefore leave the skin warm and dry, whereas natural

materials absorb water and contribute to heat debt [37]. Closer inspection of the literature

reveals a less simple picture.

In 1951, Fourt et al. [38] made a thorough and systematic investigation into the drying

behaviour of various fabrics, when the first synthetic fabrics were becoming available in

the consumer market. They characterised some key drying behaviours that seem to be

ignored in some of the newer literature. Fourt et al. found that ‘by and large’ all fabrics

dry at the same rate; however, the drying time depends on the amount of water the fabric

originally holds. This water-holding capacity depends primarily on the fabric thickness.

Forty-five years later Crow and Osczevski [37] also found a strong correlation between

fabric thickness and water pickup, and water pickup and drying time. They also presented

evidence suggesting that since the amount of liquid water picked up depends on the total

capillary volume, open-structure fabrics pick up less water. Yoo and Barker [39] found

that although hydrophilic surface treatments of hydrophobic fibres change the rate of

water absorption, they do not change the total amount of liquid water absorption. This

property is dictated by the structural characteristics of the fabric such as thickness and

porosity. Laing [40] found that whilst mass and thickness generally relate to values

obtained on the dynamic sweating hotplate, the bulk density is linked to the absorption

responses. Prahsarn et al. [41] used a novel method to show that the microclimate drying

time, or the time-dependent dissipation of accumulated moisture, is most influenced by

the pore characteristics of the fabric, rather than fabric thickness: this highlights the

importance of thin knit structures with unobstructed inter-yarn pores for optimum mois-

ture vapour dissipation. Other research suggests that hygroscopicity desirability (for sen-

sorial comfort) is dependent on whether external pressure is applied: these studies

concerned sock comfort but could be applied to other scenarios such as pressure created

by a rucksack [42,43]

The notion that natural fibres absorb more water is understandable, given their higher

regain, i.e. they can absorb more moisture vapour than synthetic fibres (but the regain of

nylon, for example, is not negligible). Crow and Osczevski found no correlation whatso-

ever between fibre regain and the amount of liquid water a fabric absorbed, and consid-

ered it erroneous to relate the two. However, Fourt et al. pointed out that one factor

influencing water uptake and drying time is the method of support during wetting and

also any mechanical treatment applied; this highlights that the method in which the fabric

is wetted is important: in Crow and Oszevski’s study, the fabrics were left overnight sand-

wiched between two wet sponges. Since there is no standard procedure for wetting fab-

rics, it can be difficult to differentiate between water absorbed in the fibres and water

entrapped between the fibres. This is somewhat different from the situation in which


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Table 1. Summary of effects of hygroscopic and non-hygroscopic fibres on thermophysiology andcomfort.

Significant differences in:

Garments compared Skin T Core T

Subjective

response

Rodwell et al. [28] Wool, terylene @ ‘ ‘

Higher with wool when moving from

31.1C, 26% -> 4.1C, 93% RH

Vokac et al. [44] Cotton, polypropylene @ ‘ ‘

Lower with cotton but insulation

also lower: 0.27 vs. 0.29

Holm�er [66] Wool, nylon @ ‘ ‘

Higher Skin Twith wet wool than

with wet nylon, only at rest

Nielsen and

Endrusick [67]

Cotton, wool, polypropylene,

polyester

‘ ‘ ‘

Only differences in sweat

accumulation were observed

Bakkevig and

Nielsen [49]

Wool, polypropylene ‘ ‘ @Wool feels ‘more itchy’ but no

difference in thermal comfort

Ha et al. [63] Cotton, polyester ‘ ‘ ‘

Sweat rates higher with polyester,

clothing T higher with cotton

Ha et al. [69] Cotton, polyester ‘ @ @Lower with cotton, attributed to

higher conductivity due to

moisture absorption

Gavhed and

Holm�er [68]Wool, synthetic @ ‘ ‘

0.3�C higher with wool

Kwon et al. [70] Wool/cotton, cotton, polyester @ @ @Differences observed in the second

exercise period with 1.5 m s�1

wind began

Gavin et al. [47] Cotton, synthetic ‘ ‘ ‘

Tanaka et al. [71] Cotton, polyester @ @ @Skin T lower with cotton; core

T lower with polyester

Roberts et al. [48] Thin synthetic, thick synthetic,

cotton

@ ‘ ‘

Skin T higher with cotton, but cotton

is twice as thick as thin synthetic

Wickwire et al. [64] Cotton, polyester ‘ ‘ ‘

–

Laing et al. [72] Wool, wool/polyester, polyester ‘ @ ‘

Smaller changes in core Twith wool,

lower skin T and better subjective

responses but not significant

Van den Heuvel

et al. [65]

Merino wool, cotton, polyester ‘ ‘ ‘


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fabrics absorb evaporated and liquid sweat in a clothing system, and may account for con-

tradictory observations made in wearer trials with human subjects.

In some wearer trials, such as those of Vokac et al. [44] (2 h intermittent exercise

at �2�C), no significant differences in sweat absorption were found between various

combinations of cotton and polypropylene two-layer clothing. However, in many wearer

trials comparing garments made from synthetic and natural hygroscopic fibres, the

hygroscopic fibres absorb more sweat. Holm�er [45] found that in wearer trials [8�C, 60%relative humidity (RH), 0.3 m s�1] wool garments accumulated an average 245 g of sweat

and nylon garments 198 g. However, it is not clear to what degree this is due to fibre

regain, since Holm�er reported that the wool garment had a slightly higher thickness and

thermal resistance. Li et al. [46] found significant differences in sweat absorption

between T-shirts of different fibres (wool, cotton, acrylic, polypropylene, polyester

(PES), acrylic, viscose, poly-cotton) at cold temperatures (14�C, 32% RH) but not in

warm temperatures (32�C, 45% RH); however, as this was not the main interest of this

study, the exact values were not reported. Gavin et al. [47] found that cotton retained sig-

nificantly more sweat than a synthetic fabric (30.5 g vs. 10.5 g) in warm conditions

(30�C, 35% RH, 3–11 km h�1). The two garments were estimated to have a similar ther-

mal insulation but that of the cotton was slightly higher (0.28 Clo vs. 0.27 Clo); therefore,

the cotton fabric may have had a slightly larger thickness. Roberts et al. [48] also con-

ducted wearer trials (20�C, 47.5% RH, 3 m s�1) and found that a ‘hot conditions’ base

layer absorbed less sweat than a ‘cold conditions’ base layer and cotton base layer: the

amount of sweat was correlated with the thickness for each fabric, but the differences

were not significant (44 g, 0.65 mm; 52 g, 1.00 mm; and 66 g, 1.20 mm, respectively).

Bakkevig and Nielsen [49] found that more sweat accumulated in wool underwear (39 g)

than in polypropylene underwear (10 g), but the wool underwear was 1.95 mm thick and

the polypropylene underwear 1.41 mm. Interestingly, in contrast to some studies, the

open-structure fishnet underwear absorbed more than the normal polypropylene under-

wear, but the thickness was more than 1 mm greater (2.45 mm). Data from Anderson

[50] regarding the perceived drying time of different garment types provides some evi-

dence supporting the idea that the open-structure ‘fishnet’ has quickest drying time: the

cotton garment was perceived to be dry in 110 min, wool in 91 min, PES garments in 55–

85 min, polypropylene in 48 min and ‘fishnet’ polypropylene in 24 min. Anderson did

not measure the initial amount of water absorbed.

Another important point may be that wool feels less wet than synthetic fibres

because more water can be bound to polar sites in the fabric structure, rather than being

present in liquid form: therefore, despite having absorbed more water it may ‘feel’ the

same as a synthetic fabric that has absorbed less moisture. This has been observed in

experiments; however, in one report this effect was only statistically significant at 25%

RH: such conditions are unlikely in real-use or inside a real multi-layered clothing sys-

tem [51,52]. In other studies, human subjects have been unable to differentiate between

different types of wet fabrics [53]. In the study of Li et al. [54], 10 male subjects wore

wool or PES long-sleeved T-shirts and acclimatised to 28�C and 30% RH. After resting

they walked at 5.6 km h�1. The wool T-shirts absorbed four times more sweat than

PES, and Li et al. argue that because of this the wool was perceived as less clammy

than PES. Finally, Fanguero et al. [55] studied the drying properties of a variety of fab-

rics incorporating wool, Coolmax and Finecool. Although it is not clear whether results

were statistically significant or not, they found that the fabric ranking of drying time

was different at normal (20�C) and high (33�C) temperatures. This could have implica-

tions for which fabrics are suitable for drying during wear (at body temperature) or for


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drying after wear (at room temperature). However, more research is required to eluci-

date this behaviour.

In conclusion, it appears that when a fabric is submerged in water (e.g. in the papers of

Fourt et al. [38] and Crow & Osczevski [37]) the amount of liquid water is mainly depen-

dent on the thickness, or in the case of open-structure fabrics, the capillary volume. The

thickness and capillary volume are most likely strongly correlated in conventional fabrics,

and therefore the correlation of water content should be made with capillary volume

rather than thickness. In wearer trials, though it is hard to separate the confounding effects

of fabric thickness and fibre type, it appears that fabrics made of fibres with higher regains

do absorb more moisture, presumably because they gain weight by absorbing both evapo-

rated and liquid sweat, and therefore will take longer to dry.

2.3. Physiological effects of the heat of sorption

Another issue extending from the water absorption and drying time debate is whether the

absorbed water has a physiological effect, either in terms of increased fabric conductivity

due to absorbed water or in terms of the heat of sorption, particularly in the case of wool,

but other fibres such as regenerated viscose also have high regain and thus the heat of

sorption [56]. Knowledge about the regain of different clothing materials was originally

important for textile traders in order to know the true weight of the product, and for tech-

nical aspects of spinning; however, the hygroscopic qualities were also expected to influ-

ence human comfort and physiology, mainly through the heat of sorption [57]. This effect

led to the recommendation of wool, a ‘minor producer of heat’, over plant or synthetic

fibres for cold weather clothing [58].

Numerical models simulating the effects of hygroscopic and non-hygroscopic cloth-

ing also suggest that hygroscopic ‘buffering’ is of little physiological significance. For

example, Farnworth et al. [59,60] argue that non-hygroscopic clothing may in fact be bet-

ter in maintaining constant body temperature, as the heat of sorption released when sweat-

ing occurs will reduce the amount of heat loss possible from the skin, at the very moment

when heat loss is desired. The model of Fengzhi and Li [61] also predicts higher skin tem-

peratures when wearing hygroscopic clothing.

Gibson [62] observed that the temperature of wool increased by up to 12�C at a con-

stant ambient temperature with the RH changing from 0% to 100%. Rodwell [28] showed

that at a constant temperature of 22.8�C when the RH is increased from 30% to 90%, up

to an 8�C rise in garment temperature is observed; however, when the increase in humid-

ity (43% to 93%) was accompanied by a decrease in temperature (20�C to 6.1�C), thetemperature of the wool garment simply minimises the size of the clothing temperature

drop (by 1.5�C compared to a synthetic terylene garment).

Table 1 shows the main results of studies with human subjects regarding hygroscopic

and non-hygroscopic clothing in chronological order. At first glance, it is clear that neither

hygroscopic clothing nor non-hygroscopic clothing has significant influence on human

thermophysiology.

The studies of Ha et al. [63], Gavin et al. [47], Wickwire et al. [64] and Van den

Heuvel et al. [65] found no significant differences in skin or core temperature or subjec-

tive responses. These studies were all conducted in warm conditions: (37�C, 60% RH,

0.1 m s�1); (30�C, 35% RH, 3–11 km h�1 wind); (35�C wet bulb temperature); and

(41.2�C, 29.8% RH, 4 km h�1 wind), respectively. Participants in these studies rested for

90 min; rested, ran, walked and recovered for a total of 60 min; performed simulated

industrial tasks for 120 min; and walked for 120 min before performing an alternating

running–walking protocol, respectively. In the latter two studies of Wickwire et al. and


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Van den Heuvel et al., the synthetic or natural underwear was worn beneath additional

military wear and body armour.

Other studies by Rodwell et al. [28], Vokac et al [44], Holm�er [66], Nielsen and

Endrusick [67], Gavhed and Holm�er [68] and Roberts et al. [48] found differences only

in the skin temperature due to the heat of sorption, but no significant changes in core tem-

perature or subjective response. Rodwell et al. found slightly higher skin temperatures

with wool in very specific conditions (sweating slightly at rest in a hot dry environment,

31.1�C, 26% RH, moving into a cold damp environment, 4.1�C, 93% RH). Holm�er onlyfound significantly higher skin temperatures with wet wool compared to wet nylon when

at rest in cool temperatures (8�C, 60% RH, 0.3 m s�1), while Gavhed and Holm�er foundhigher skin temperatures with wool compared to synthetic (�0.3�C) during exercise in

cold conditions (�10�C, 100% RH, 0.2 m s�1). The results of the studies of Roberts et al.

and Vokac et al. are hard to interpret due to differences in fabric thickness. For example,

in neutral conditions (20�C, 47.5% RH, 3 m s�1 wind), Roberts et al. observed higher

skin temperatures with cotton underwear than with thin synthetic underwear, but the

thickness of the cotton underwear is almost double that of the synthetic (1.20 mm vs.

0.65 mm). Vokac et al. admit that the systematically lower (1�C) back skin temperature

observed with the cotton garment in their study cannot be definitively attributed to fibre

hygroscopicity as the thickness and thermal resistance of the garment were lower

(2.6 mm vs. 3.2 mm; 0.41 Clo vs. 0.62 Clo).

The studies of Ha et al. [69], Kwon et al. [70], Tanaka et al. [71] and Laing et al. [72]

found significant differences in skin or core temperature or subjective responses with nat-

ural and synthetic fibres. With the exception of the study of Tanaka et al., these studies

found that the core and skin temperatures were lower and subjective responses more

favourable when subjects exercised in garments made from natural fibres. Kwon et al.

and Ha et al. attributed these differences to the fact that natural fibres absorb more mois-

ture, and therefore their thermal conductivity increases by a greater amount, as the ther-

mal conductivity of water is nearly 20 times greater than that of most textiles. In the

study of Kwon et al., significant differences were only observed when 1.5 m s�1 wind

was introduced, after the garments were wetted with sweat.

In the study of Tanaka et al. [71], the effects of PES and cotton garments on subjects

immersing their legs in water of increasing temperature were investigated: they found

that the skin blood flow, skin–clothing microclimate and clothing surface temperature

were higher with cotton than PES and these physiological effects were accompanied by

less comfortable sensations. The authors suggest that because of their method where heat

stress is applied to totally stationary participants, heat of sorption effects are only due to

sweating, and changes in clothing fit and body movement induced air flow do not mask

these effects, making their method superior.

Confounding factors often make it difficult to reach solid conclusions about the differ-

ent effects of fibre type. For example, Wang et al. [73] compared two four-layer clothing

systems [‘traditional’ and ‘moisture management function’ (MMF) clothing]. Participants

walked on a treadmill at �15�C. Wang et al. found that the MMF clothing resulted in

lower humidity next to the skin and that the sweat produced was more effectively trans-

ferred to the environment with hygroscopic clothing. The difference between the MMF

and traditional clothing appears to be that the inner layers of the vest and coat were made

from a wool–cotton blend rather than nylon; however, the air permeability of these fabrics

differed by a large degree (<0.02 vs. <78.80 ml s�1 cm�1 at 100 Pa). Therefore, it is dif-

ficult to conclude which of these physical parameters (air permeability or fabric regain)

constitutes ‘moisture management’. In the study of Guo et al. [74], two types of fabric

were compared: PPE1 consisted of a cotton scrub suit worn inside a 100% polyethylene


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‘barrierman’ and PPE2 consisted of a waterproof breathable gown and a scrub suit

‘treated to allow moisture to be transferred away from the skin to the surface of the gar-

ment, where it could evaporate quickly to keep the body dry and comfortable’. Whilst the

latter, PPE2, did indeed improve thermophysiological comfort, the design of the two gar-

ments was not identical: PPE1 was a relatively tight fitting one-piece suit and PPE2 was a

more loose fitting smock ending just below the knees, which would tend to encourage

heat and mass transfer by ventilation [75]. The respirator design was also different. Such

confounding variables again make it difficult to confirm the efficacy of ‘moisture man-

agement’ underwear. Confounding factors exist not only in terms of fabric parameters,

but also in terms of climate and activity type. The effects of hygroscopicity will be very

different if the body produces water vapour (insensible perspiration) or liquid sweat

(sensible perspiration).

Models and laboratory testing often propose that hygroscopic or non-hygroscopic

fibres will have a significant effect on human comfort [76], but this is not always evident

in wearer trials. Therefore, it appears that factors such as air layers, perception insensitiv-

ity of human subjects and physiological variability mean that especially in hot environ-

ments, fibre hygroscopicity does not have such a large impact on human

thermophysiology and comfort as might be expected from properties measured in labora-

tory tests. Barnes and Holcombe [77] discuss the reasons why fibre sorption may be

important in the laboratory but not in real wearing conditions: one reason is that the mag-

nitude of vapour resistance can be made deliberately large to investigate how well a

model describes sorption effects, but in real-life the magnitude is smaller and thus the

sorption effects are not as important. They go on to explain that the effects of air move-

ment and the wetting of clothing are likely to outweigh sorption effects. This is demon-

strated in the work of Stuart et al. [78] who first dried the wool garments in their study to

maximise the heat of sorption, which were mittens in direct contact with the skin. This

demonstrates that the hygroscopic qualities of wool can be manipulated to provide com-

fortable sensations, but this comes at the cost of a considerable effort for the user, which

may be unreasonable to expect in normal use. Furthermore, laboratory testing has also

shown that differences present in one-layer fabric tests may be nullified when the same

fabric is tested as part of a multi-layer system [79]. Farnworth and Dolhan [80] also found

that differences between polypropylene and cotton underwear were too small to have any

influence on thermal comfort. Lotens and Havenith [81] conclude that in the event that

absorbing clothing is perceived as being more comfortable than non-absorbing clothing,

this is most likely due to tactile properties and differences in liquid management than

because of any effect on body heat transfer. For example, wool fibres with small fibre

diameters generally elicit positive comfort sensations [82]. There is ongoing debate about

the role of fibre hygroscopicity with regard to the ‘post-exercise chill’. Though Holm�er[66] showed that participants resting in wet wool garments were slightly warmer, other

unpublished data shows that participants resting in wet cotton garments (vs. PES) suffered

from a more pronounced post-exercise chill. Therefore, it may be the case that for wool,

the heat of sorption outweighs the increase in fabric conductivity when wet, but for cotton

this is not the case. Further work is required to clarify the exact behaviour.

2.4. Fabric structure

So far, the main focus of this section has been the fibre type of base-layer garments and

the relative merits of wicking and wetting properties. However, some authors claim that

the knit structure of underwear is of far more importance than material fibre type [83]. In


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particular, the differences in regular underwear and mesh, or ‘fishnet’ underwear, are dis-

cussed (see Figure 1). The advantages of open-structure underwear in drying time and

transverse wicking have already been addressed.

Roberts and Bertram [84] describe the ‘proper’ use of mesh underwear: during exer-

cise, outer garments are loosened and unfastened allowing air circulation in the open net-

work of the vest. After exercise the outer layers are fastened and sealed with ‘a sweat rag

or handkerchief’, which produces a stable, insulating layer of air next to the skin. In espe-

cially cold weather, a second open-structure Brynje garment was used between other

clothing layers. Fonseca [85] showed with a stationary manikin and low external air flow

(e.g. similar to a resting person) that in terms of thermal insulation, mesh underwear

behaves very similarly to conventional underwear.

Nielsen & Endrusick [83] tested five different types of underwear, including fleece,

one-by-one rib-knit and fishnet, and found evidence to support the notion of air circulation

in mesh underwear improving thermal comfort. They found that the physiological

responses in human wearer trials were not correlated with laboratory measures of thermal

and water vapour resistance. Instead, they found that the underwear with most open struc-

ture, namely fishnet, resulted in the lowest skin temperature and skin wetness when worn

as part of a three-layer clothing system during 40 min cycle exercise at 5�C (dew point

temperature �3.5�C). They attribute these differences to the open structure that allows air

to ‘sweep directly over the skin’. Goldman [86] reports that in physiological trials in 1944,

the string vest had a higher level of ‘acceptability’ in terms of subjective thermal comfort

perceptions. Ueda et al. [87] studied underwear garments including a mesh base layer and

in some cases recorded lower temperatures at the chest with the mesh base layer.

Open-structure fabrics may also represent an advantage once the underwear has

become wet. In the study of Bakkevig and Nielsen [88], 10 men rested in wet underwear

for 1 h in cold conditions (10�C, 85% RH, 0.1 m s�1). Conclusions about the influence of

fibre type were confounded by large differences in fabric thickness, but the authors con-

cluded that the thickness of the underwear had more influence than fibre type, and that a

fishnet structure might have an advantage as there is a less area in contact with the skin:

this was evidenced by higher skin temperatures and more positive subjective responses.

Figure 1. Brynje fishnet ‘mesh’ and regular underwear.


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Critchley also recommends Brynje for heavy exercise in cold climates [89], and

reports further benefits of Brynje in hot weather: specifically, by preventing a sweat-satu-

rated shirt from clinging to the skin, the occurrence of mosquito bites can be reduced.

Woodcock [90] also notes that in cold weather, the air gap adjacent to the skin created by

Brynje decreases heat loss from the body when exterior clothing is wet from sweat or

rain. Such air gaps decrease the cooling effect of wet fabrics on the skin [91].

2.5. Transient effects

Kerslake [92] suggested that underwear has a further function, which is to reduce skin

cooling caused by momentary contact with cold outer fabric layers. Even very thin under-

wear can create the impression of keeping the skin warm, because cold receptors in the

skin are especially sensitive to transient cooling events. Hes et al. [93] suggest that thin

plain knit underwear reduces the thermal impact of a cool outer fabric by about 43% and

thicker rib-knits or PP knits with higher thermal resistance may reduce the effect of ther-

mal absorptivity of the outerwear by up to 11% of its original value.

2.6. Conclusions and future trends

The literature suggests that the base-layer performance is far from simple and depends on

a complex interaction of physiological and environmental factors. It appears that the con-

clusions made 60 years ago by Andreen et al. [94] that comfort largely depends on fabric

geometry and construction and the manner in which the fabric is worn on the body (i.e.

garment design, see Nielsen et al. [95]) still stand. That is to say that macroscopic differ-

ences are more important than microscopic differences (e.g. fibre type), and differences

in thermophysiological response to different fibre materials are either non-existent or sim-

ply difficult to detect.

Essentially, the desirability of factors such as wetting and wicking ability, drying time

and hygroscopicity depends on the application. For example, good wetting and wicking

(transverse and planar), especially with thick underwear, may reduce the efficiency of

evaporative cooling (sweating): for a resting person with wet skin looking to conserve

heat, this will be a desirable property. However, for an exercising person that may poten-

tially overheat, it would be more advantageous for the sweat to evaporate from the skin.

Given these conflicting requirements, the optimal situation may be to have a fabric with

‘switchable’ wicking characteristics: maximising sweating efficiency when required and

moving water away from the body by wicking when it is not required. For example, surfa-

ces that switch from hydrophilic to hydrophobic with temperature could allow the wick-

ing of water away from the cold skin, but allow sweat to remain on the surface of the hot

skin [96–98]. Alternatively, it has been suggested that rather than one type of base layer

being optimal for all conditions, outdoor activists should consider using different types of

base layer for different activities [99].

From a water absorption and drying perspective, the literature suggests that a very

thin, synthetic fabric (low regain) with an open structure should absorb the least water

(when wetted by a mix of liquid and evaporated sweat) and dry most quickly. Of course,

other practical requirements such as protection from sunlight and social modesty must

also be considered.

The role of hygroscopicity in underwear has also been addressed. Conclusions are

hard to draw from the conflicting evidence in the literature, but it appears that depending

on the environmental conditions and exercise type there may be some factors to consider


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when choosing natural or synthetic fibres. Particularly for wool fibres, it appears that the

high regain and heat of sorption mean that at rest, when the underwear is wet, there is a

small heat of sorption effect that may result in marginally higher skin temperature, though

these are usually imperceptible to the wearer. When exercising in warm conditions, the

high regain or water absorption of natural fibres appears to reduce the insulation of the

clothing, thus cooling the wearer: these effects are not likely to be significant when worn

as part of a clothing ensemble or in a highly stressful hot environment. There is some

speculation that the heat of sorption caused by sweat absorption increases heat stress on

the wearer: this effect has been observed but only in conditions where the subjects are still

but sweating; in such situations, fibres with high regain and heat of sorption should be

avoided.

In conclusion, choosing the ‘correct’ base layer requires a careful consideration of the

exact environmental conditions (or changes in these) that are expected to be encountered,

and the type of exercise performed. In the future, advances in technology may create

materials that can intelligently change their wetting, wicking, drying (porosity) and

hygroscopic qualities to most benefit the user in the specific scenario they encounter.

3. The mid layer

The primary role of the mid layer is to provide insulation. Mid layers are commonly

based on synthetic or natural fibre knitted fleece structures. Synthetic and natural non-

woven battings, or down fibres, are also sandwiched between tightly woven materials to

create mid layer garments. The mid layer functions by trapping still air which has a low

thermal conductivity (0.025 W m�1 K�1). Due to this, most textile materials have a simi-

lar specific conductivity (0.042 W m�1 K�1); the reason this is different from that of still

air is due to heat transfer by radiation, and with dense fabrics fibre conductivity may also

play a role [100]. Due to this simple relationship between clothing insulation and the

thickness of trapped still air, it is often stated that with regard to dry heat transfer, clothing

can consist of any material that can create still air layers. For example, Holm�er [101]states that ‘choice of textiles is more or less arbitrary as long as good insulation is

provided’. Goldman [102] states that ‘if the measured value (of thermal insulation) is not

1.57 Clo cm�1, repeat the thickness and Clo measurement, since either one or the other

measurement is wrong’.

There are of course exceptions to these statements. In 1948, Backer [103] observed

that because textiles are low-density structures, containing a large proportion of air, the

thermal resistance of textiles is primarily dependent on the behaviour of the entrapped air

layer, i.e. although the density of conventional textiles is such that they resist free and

forced convective flow [104], low-density insulation can exhibit large decreases in ther-

mal resistance with increasing air flow. Conversely, fabrics with high air flow path tortu-

osity [105] and s large fibre surface area [106] minimise air movement and therefore

convective heat loss. Goldman also states that unusually low-density fabrics will cause

exception to his rule by admitting more heat transfer by radiation. Holm�er expands andadds that with sweating and long outdoor excursions, the low hygroscopicity of synthetic-

based mid layers becomes important, whereas moisture absorption in natural fibre mid

layers decreases the thermal resistance of the clothing, and particularly when the meta-

bolic rate decreases after exercise, ‘post-exercise chill’ presents a threat.

Since the effect of clothing hygroscopicity has been discussed previously, this section

focuses primarily on the effect of air movement on insulation.


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3.1. Air movement and insulation reduction

In 1938, Winslow et al. [107] showed that the convection constant for clothed humans,

K, varies with the square root of the air velocity, V (cm s�1):

K ¼ 1:043ffiffiffiffiV

p:

In 1946, Rees [108] found a similar relationship between thermal resistance of an air layer

R (in TOGs) and air velocity V (miles h�1, at sea level):

R ¼ 1

0:42þ 0:81ffiffiffiffiV

p :

Fourt and Hollies [109] also found that the insulation of the clothing boundary layer,

R, was dictated by the air velocity V raised to the power 1/2 (equal to the square root):

Boundary insulation R ¼ 1

0:61þ 0:19V 1=2:

It is also possible to analyse decreases in insulation as linear increases in conductance,

and some authors claim that this approach is advantageous [110]. Givoni and Goldman

[111] saw that the effective wind speed (Veff) responsible for reducing clothing insulation

was a combination of the wind speed (V) and body motion (linked to the metabolic

rate M) and used the following equation to describe these effects where increases in

metabolic rate cause a linear increase in effective wind speed:

Veff ¼ V þ 0:004ðM � 105Þ:

Havenith et al. [112] also observed that changes in activity (similar toM) and posture can

be described linearly, whereas changes due to wind are described by a square root func-

tion. More recently, empirical models combining the effects of walking, wind speed and

air permeability of the outer layer of clothing have been developed [113]. First, equations

incorporated the effects of walking and wind speed [114]:

Insulation reductionð%Þ ¼ 0:858e�0:05V�0:161w;

where V is the wind speed and w is the walking speed, in m s�1. Such equations are also

available for nude and dressed subjects [115]. Other studies have shown that the reduction

in insulation due to wind is related to fabric air permeability [116]. Correction equations

for insulation, It,r, dependent on body movement, wind and clothing air permeability have

been published by Nilsson et al. and Holm�er et al. [117,118] and subsequently expanded

by Havenith and Nilsson [101,119] with the form (see also Figure 2)

It;r ¼ e½�0:0512�ðV�0:4Þþ0:000794�ðV�0:4Þ2�0:0639�w��p0:1434h i

� It;static;

where V is the air velocity, w is the walking speed and p is the air permeability in

l m�2 s�1. Depending on the value of It,static, different correction equations should be used

(for insulation from 0 to 0.6 Clo, 1.22 to 1–84 Clo and 1.49 to 3.46 Clo): more details can

be found in Havenith and Nilsson’s publication [119].


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The combined effect of relatively modest walking (0.77 m s�1) and wind speed

(1.0 m s�1) has been shown to have heat exchange capacities of up to 161 W m�2 by

Bouskill et al. [120]. Studies comparing the magnitude of the bellows effect with moving

thermal manikins and human subjects have concluded that due to the greater complexity

of human movement, ventilation and therefore heat transfer is greater with human

subjects [121].

3.2. Separating the effects of air movement on clothing insulation

Body movement (and potentially body heat, in the case of natural convection) and exter-

nal air movement reduce the total clothing insulation in various separate ways. These are

summarised in Figure 3 and discussed in more detail in the following section.

3.2.1. Reduction of boundary layer insulation

Air layers form on the surface of textile layers by adhesion of the gas molecules to the

surface; further layers are formed by a second layer of molecules adhering to the first,

and so on [122]. In still air this boundary layer may be up to 12 mm thick but with vigor-

ous motion, reduced to 1 mm. Backer [103] suggested that the reduction of the boundary

layer insulation of textiles is the dominant mechanism of clothing insulation reduction up

to air flows of 2.7 m s�1. Yankelevich [123] made a theoretical appraisal of the effects of

air flow on clothing. He stated that in the case where the outer layer is totally imperme-

able, reductions in thermal resistance are due only to the reduction of the boundary layer

(assuming the insulation is not compressed). As well as the external air flow (wind) affect-

ing the boundary layer insulation, body motion can erode the boundary layer. Nielsen

et al. [124] found that the insulation of the boundary layer decreased by 7%–26% when

bicycling and by 35%–45% when walking. Where body parts are not covered by clothing,

convective heat transfer is higher [125], and the local thermal resistance of such

Figure 2. Nilsson and Havenith’s relationship for thermal insulation, body movement, wind and air

permeability.


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non-covered body parts can be affected by clothing modifications in other areas. For

example, using a hood stabilises the air around the neck and torso, increasing the thermal

resistance in these areas [126]. Regional convective cooling of clothing has also been

studied using thermal imaging techniques [127].

3.2.2. Body motion

In addition to eroding the boundary layer insulation, body movement creates air move-

ment inside the clothing microclimate. Vokac et al. [128] observed the effect of body

movement induced air flow on physiological parameters; they observed that during exer-

cise, the skin temperature, humidity and heat flow reach a new, high steady state, and

upon cessation of exercise an additional sharp increase in skin temperature and humidity

and a decrease in heat flow occur which was taken as evidence of ‘the bellows effect’,

by its absence. Fanger [129] states that walking motion creates relative air velocities of

1–2 m s�1. Using a mass-loss technique, Nishi and Gagge [130] also found that when

walking the average air velocity on the surface of the nude human body was between 1

and 1.91 m s�1, and that the air speed is highest at the extremities. In 1947, Belding [131]

found that the air movement created by body motion could reduce the thermal resistance

of a clothing system by up to 50%. Nielsen et al. [124] also investigated other activities,

finding that bicycling and walking reduced the intrinsic clothing insulation (excluding

changes in the boundary layer) by 30%–50%. Changes in insulation due to body move-

ment are larger with thick fabrics and large air layers [112].

Figure 3. Illustration of the separated effects of ventilation.


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3.2.3. Interaction of wind and body motion

The effect of body motion eroding the outer clothing boundary layer is reduced in windy

conditions [132]. Nielsen et al. [124] also found that once the surface layer of air is dis-

turbed by an external air flow, further disturbances caused by body motion have only a

small insulation-reducing effect.

3.2.4. Free convection

In environmental conditions with still air, free convection, as opposed to forced convec-

tion, is dominant [129]. With regard to fabrics and the layers of air present in the clothing

system, Backer [103] suggests that free convection occurs in air layers larger than 1/200

(12.7 mm), Spencer-Smith [133] states that natural convection is negligible in air layers

smaller than around 8 mm, and Farnworth observed the onset of natural convection at

13 mm [134]. Usually the air spaces in textiles are much smaller than this, so natural con-

vection is limited, but Cena and Monteith [135] have estimated that the ‘slow ascent’ of

free convection in animal furs proceeds at 0.01 m s�1. The authors therefore conclude

that this effect is ‘trivial’ in relation to the velocity of external air flows. Chen et al. [136]

and Fan and Qian [137] have shown that whilst larger garments trap more air and are

therefore warmer, above a threshold natural convection occurs in the air spaces, and ther-

mal and water vapour resistance begins to decrease.

3.2.5. Compression effects

In high winds, clothing systems are ‘squashed’ or compressed by the air flow, changing

the thickness of the textile and air layers [123]. Backer [103] suggests this starts occurring

at air flows of 2.7 m s�1 and above, and is particularly prevalent with open-structure

‘under layers’. In some studies, compression effects have been eliminated with the use of

rigid metal fly sheets [138]. Compression effects also occur with changes in posture such

as sitting, and looser garments show larger changes in insulation because the relative

changes in geometry are larger [125]. Anttonen and Hiltunen [139] observed that com-

pression of clothing is more pronounced with laminated air-impermeable fabrics which

may be compressed up to 45 mm at the windward side; more permeable fabrics are com-

pressed only by 10–35 mm. Stevens [15] also observed that with laminated fabrics, warm

air from the microclimate is removed when gusts of wind momentarily compress the

clothing. Other changes in geometry also occur due to wind, in particular inflation at the

rear of the body [139], where inflated air layers of 60–70 mm can occur. Due to such

compression effects, Fan [136] concludes that tighter fitting or tailored garments are pref-

erable in keeping the body warm in cold and windy conditions. 3D spacer fabrics have

also been recommended to avoid compression-induced insulation loss [140].

3.2.6. Penetration of fabric layers

When the outer layer of a fabric system is permeable, wind can penetrate and disturb the

inner air layers, reducing the system’s thermal resistance: in non-uniform air flows, this

effect causes local differences in thermal resistance [141]. Using both the heated cylinder

and the hot plate methods, Niven [142,143] found that windbreaks (outer layers with low

air permeability) are, quite intuitively, most effective outside the insulation layers, and

are of special importance for open-structure materials (in this case, an open-cell urethane

rubber). Niven also recommends that clothing designed for windy weather should include


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an additional exterior windbreak layer. The use of linings to stabilise air in insulating bat-

tings has also been suggested [144]. In some models of clothing insulation, windproof

outer layers are given double-weighting [145]. Fonseca and Breckenridge [138,146] pro-

posed a novel solution to the problem of outer-layer wind penetration, based on the fact

that at the time of writing, the manufacture of extremely impermeable outer layers was

problematic. Their solution was to provide an air gap or channel that directed the air flow

circumferentially around the human trunk, rather than penetrating the inner layers of insu-

lation. They also showed that fabrics with different air permeability exhibit different

threshold air flows where convective cooling becomes significant, and demonstrated with

human trials that their concept of a ‘bypass’ layer could eliminate the requirement for

absolutely air impermeable outer layers. Kind et al. [144] also explored the Fonseca

bypass layer concept, measuring dynamic pressure on a heated cylinder, on the premise

that using bypass layers is desirable as air-permeable outer layers allow increased water

vapour transfer. They suggest using multiple windbreak sheaths and permeable batting

layers to reduce the dynamic pressure penetrating the clothing. In further work, Kind and

Broughton [147] used a plastic mesh to create a ‘bypass’ layer: such bypass layers could

potentially be created with knitted 3D spacer fabrics. The concept of a threshold perme-

ability where moving air penetrates the fabric was also observed by Lamb and Yoneda

[148] using a rotating heated cylinder; the penetration pressure was found to depend on

the fabric permeability to the power approximately 2/3. Wind penetration can also depend

on the relationship between the fur or fabric geometry and wind direction, e.g. heat loss

through newborn caribou furs is more than doubled when the air flow is parallel to the

hair axis rather than perpendicular to it [145].

3.2.7. Filtration and ventilation

When the clothed human body, often modelled as a cylinder, is exposed to wind, an

uneven pressure builds up on the outer surface (see Figure 4) [123]. If the external

clothing layer or apertures allow air penetration, air will filter through the clothing from

areas of maximum pressure to areas of minimum pressure, thus filtering both in and out

of the clothing system. This effect is also described by Stuart and Denby [149] who pre-

dict that air penetration occurs at primarily 30� either side of the windward stagnation

point, and air is ‘sucked’ from the leeward wake area. Yankelevich states that the degree

of filtering is larger with loosely fitting clothing. It is important to note that in addition

to this mixing of ambient and microclimate air, convective cooling occurs due to inter-

nal circulation and movement of air, which would occur even in a perfectly sealed cloth-

ing system [125].

Crockford [75] was the first to adopt trace gas techniques (summarised by Lumley

[150]) to assess the effects of fit garment design, fabric permeability, body movement and

wind on ventilation. In accordance with Yankelevich [123], he found that looser fitting

jackets, open apertures and shorter cut jackets (apertures closer to the main trunk of the

human body) all increase ventilation; these conclusions are supported by den Hartog’s

work [151]. Increasing the ventilation rate is also possible by increasing fabric air perme-

ability [152] or by adding foam spacers to the clothing (the latter modification can more

than double ventilation rates) [153]. Danielsson [154] also measured increased convective

heat loss coefficients in clothing with larger free air spaces. In terms of ventilation in air

spaces caused by body motion (movement of fabric relative to body), Satsumoto et al.

[155] found that there was an optimal air space of 10 mm for heat transfer, and heat trans-

fer was reduced with air gaps of 30 mm. Birnbaum and Crockford [132] developed the


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technique and measured ventilation rates of 15–115 l min�1 in various types of foul

weather clothing. They estimated that 300 l min�1 would be desirable for hard labour.

Lotens [153] estimated that 450 l min�1 is desirable, but 360 l min�1 is perhaps maximum

available ventilation, meaning that fabric properties are also important. Tracer gas techni-

ques have also been applied to cylindrical columns where ventilation rates of fabric sys-

tems were found to increase linearly with air speed and in a more pronounced fashion

with highly-air-permeable fabrics [156]. Ueda et al. [87] also performed local ventilation

measurements and found ventilation to be highest at the chest, followed by at the back

and arms. It is also possible to estimate the ventilation rate using thermal manikins [157].

Interestingly, the modelling work by Ghaddar and Ghali [158–162] assumes that the

‘body trunk does not swing during walking’, and they therefore conclude that trunk venti-

lation is not as large as limb ventilation. Vokac et al. [128] suggest that there is in fact bel-

lows ventilation caused by the rotation of the trunk and changing size of clothing air gaps,

particularly at the back. This bellows ventilation mechanism may account for

the discrepancy between the theory of Ghaddar and Ghali and the measurements of

Ueda et al.

Figure 4. Air flow characteristics around a human-like cylinder.


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3.2.8. Openings and apertures

Openings and apertures in the clothing, whether for ergonomic purposes (neck, wrist and

waist openings) or specifically designed to increase the cooling effects of air flow (e.g.

chest, back and pit-zip openings), provide avenues for ventilation and additional heat

loss. In 1965, Fonseca and Woodcock [163] investigated a model clothing system which

featured an opening; in this case, the aim was to simulate a hood. They found that heat

transfer through this opening was higher with a looser fitting fabric and could be limited

with a fur lining or ‘ruff’ that stabilised the air around the opening. Lotens and Havenith

[153] found that typically vents increased ventilation by 40–60 l min�1. Vents can be con-

sidered to have the same effect, albeit localised, as air-permeable outer fabrics, i.e. they

allow wind to penetrate and disturb the inner layers of the clothing system. Danielsson

[154] used heated thermistors to measure the air speed inside the clothing microclimate

and found that the internal air speed was 30%–40% higher with apertures opened in high

winds. Ueda and Havenith [152] also showed that ventilation rates are higher with open

clothing apertures at the wrist, neck, etc. Ruckman [164] studied the effect of openings of

outdoor clothing and found that they could make small reductions in skin temperature.

The largest opening was the most effective. Ruckman found that during rest the jacket

fabric had more importance in maintaining comfort than openings. Satsumoto [165]

found that in some cases one opening allowed more heat transfer than two openings, and

the air velocity was double with one opening than with two. In the same way as body

movement and wind interact such that their combined effect is less than the separate

effects, vents provide smaller improvements when they are close to openings, e.g. at the

waist [153]. Satsumoto [155] showed that heat transfer, and also air velocity [166], is

already largest nearest openings, so further increases in ventilation cause only a small dif-

ference. Fanger [129] points out that local convective cooling can be pleasant or unpleas-

ant depending on whether a person initially feels too warm or too cold.

3.3. Air flow effects in laboratory tests

The thermal and water vapour resistance reducing properties of air flow have long been

observed in laboratory-based fabric-testing methods; in 1947, Fourt and Harrist [167]

noticed that the water vapour resistance of an open-structure mosquito net fabric was

extremely low, which they attributed to air penetration. More recently, Gibson [168] iden-

tified that when measured with a guarded hot plate, cover fabrics on insulating materials

(especially open foams) dramatically increase the thermal resistance of the insulation,

indicating that air penetration causes large decreases in thermal resistance. Gibson identi-

fied that effects of air penetration are dependent on the thickness, pore size and air-

passage tortuosity. Dyck et al. [169] also recognised that air penetration of samples in

guarded hot plate thermal resistance testing can cause large differences in results which

can be problematic when comparing results from different laboratories. With extremely

high air permeability spacer fabrics, tested to the ISO 11092 standard [170], the thermal

resistance is no longer dependent on the thickness due to air penetration, as shown in

Figure 5.

3.4. Materials vs. design

Crockford [75] recognised that the design of garments, particularly those designs which

encourage ventilation, is responsible for allowing relative comfort during hard work in


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adverse conditions with impermeable fabrics. Other authors have recognised the signifi-

cance of ventilation [171] and stressed the importance of clothing designs which enable

air penetration and subsequent heat removal. Lomax [106] states that WBFs should only

be regarded as one component in the overall moisture management system, and suggests

that factors such as moisture buffer layers, and ventilation features, are of equal, if not

more, importance.

3.5. Open-structure insulation

Particularly where large thermal gradients exist between the body and the environment,

heat transfer by radiation can be significant in foam structures [141] and also low-density

felted and knitted fabrics [172]. In thermal protective clothing, the insulating properties

of a fabric diminish with increasing air permeability [172,173]. Some innovations have

attempted to take advantage of this property (air flow dependence) of low-density insula-

tion. For example, Spink [174] has patented a garment based on a specially shaped low-

density polyurethane (PU) foam mid layer and a ventilated air-impermeable shell. In a

product evaluation for Salomon, a 3D mesh insulation top was tested, but participants felt

cold because the outer garment was permeable to air [175]. Krel et al. [176] found the

flocked fabrics allow greater heat fluxes and felt that this may, if incorporated into sports

clothing, increase labour productivity and influence the psychological and physiological

thermoregulation system, but did not test these claims. Recently, Helly Hansen has

released a fleece garment designed to increase ventilation, whereby the fleece fabric has

circular holes of different sizes removed from the fabric [177].

3.6. Synthetic and down insulation

As with base-layer garments there is considerable debate about the various pros and cons

of different types of insulation. As mentioned in the introduction, the dry thermal resis-

tance of different materials differs very little and so of more importance is the way in

which different fibres interact with water, and practical considerations such as weight and

thickness required with different fibres. The different effects of water are the heat of sorp-

tion (which in Section 2 has been shown to be of little physiological significance),

Figure 5. The relationship between thermal resistance and thickness for fleece and spacer fabrics.


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absorbed water decreasing insulation (thought to be higher in the case of natural fibres

coupled with slow drying times) and the effect of water on the structure of the insulation

(i.e. collapsing in the case of down insulation). It is commonly stated that synthetic insula-

tion does not absorb water but in reality the situation is more complicated and deserves

closer inspection. The interaction depends on how the fabric is wetted: if synthetic and

down insulation are submerged in water and compressed, then naturally the inter-fibre

voids become filled with water. If the insulation is threatened with wetting by rain, then

the dependence is on the waterproofness or water repellence of the outer layer. Evapo-

rated sweat may also condense inside the insulation. In each case, the primary difference

is that synthetic insulation retains its loft when wet, whereas the structure of down collap-

ses and the down fibres can ‘clump’ together [178]. In this case, the synthetic insulation

will retain more of its insulation than the down. There are also differences in the durabil-

ity characteristics of down and synthetic insulation. Whilst the durability of down is

excellent in terms of repeated recovery from compression (resilience) [179,180], the dura-

bility as a garment is perhaps vulnerable, since in the case of the outer fabric of an insu-

lated garment being damaged, the fibres are free to escape, whereas the synthetic batting

is not. Another consideration is the weight and thickness requirements of jackets made

from synthetic or down insulation: down provides better insulation per mass and synthetic

better insulation per thickness (due to the radiation-blocking density) [178]. Therefore, a

garment required to provide 3 Clo of insulation could be 1 mm thinner if made from syn-

thetic insulation, a relatively small difference probably unnoticeable to the wearer,

whereas the down garment filling would require approximately half the mass (�200 vs.

400 g, calculated from data in [181]).

A further detail regarding down insulation concerns down ‘fill power’, a measure of

the loft (volume occupied by a certain mass of down, usually given in cubic inches per

ounce) of a down product [182]. There are a number of different standards for measuring

loft, the two most pertinent being the US Federal test and the standard Lorch test, which

is approved by the International Down and Feather Testing Laboratory (IDFL): the IDFL

test yields results approximately 4% more conservative than the US test [183]. In a practi-

cal sense, higher fill power results in a lighter garment, as less down is required to create

the same loft, and therefore the same, or similar, insulation. Currently, PHD Mountain

Software offers 900 fill power down, and Patagonia’s Encapsil plasma treated water resis-

tant down is claimed to have a fill power of 1000. Another topic is down ‘overstuffing’,

i.e. adding more down than necessary based on its fill power and the volume of garment

necessary to fill. Whilst this probably decreases the insulation per mass, overstuffing may

be useful to minimise the chances of dead spots where no down is present, and in main-

taining loft in areas where garments are compressed.


This section has addressed insulation, which functions by trapping still air layers. Due to

this fact, when body movement or wind changes the characteristics of these air layers, the

clothing insulation also changes. By considering the separate mechanisms by which these

reductions in clothing insulation occur, clothing designers may be able to create clothing

with ‘flexible’ insulation which can be adjusted to suit metabolic heat output, and avoid

unwanted heat loss due to wind. The wearer can also effect large changes in clothing insu-

lation by using clothing openings.

The insulation per mass provided by down is difficult to improve upon, as its fibre

radius and fractal structure, studied with numerical modelling and other approaches,


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appear to be optimal [184–187]. However, many innovations have recently emerged to

improve its water repellence. Some examples are ‘Dri down’, ‘Down Tek’ and ‘Encapsil’

– the latter being a plasma-applied coating. There is also a focus on improving the air per-

meability of insulation, e.g. Polartec ‘Alpha’ insulation and the aforementioned Helly

Hansen ‘H20 flow’, a fleece with die or laser cut holes. Gibson et al. [188] found that

although nanofibres did not seem useful for high loft thermal insulation, they may play a

role in improving the performance of hybrid insulation with high bulk density. Plasma-

deposited metal layers may be useful in reducing radiative heat loss in low-density insula-

tion with better durability and negligible impact on evaporative resistance as was the case

with earlier reflective coatings [189–191]. Aerogels have very low thermal conductivity,

but it is difficult to incorporate them into fabrics and clothing, as in order to overcome

their brittle qualities they must be supported by fibres, and the amount of fibres required

means that they paradoxically carry a weight penalty compared to other insulating materi-

als, and cannot achieve comparable thermal conductivity at low bulk densities [188].

4. The shell layer

The role of the shell garment is to protect the inner layers from the ingress of wind and

rain, and provide mechanical protection to the whole clothing system. It should simulta-

neously allow the transfer of water vapour (evaporated sweat) from the clothing microcli-

mate to the environment. Such fabrics are known as WBFs. WBFs are typically divided

into four categories: tightly woven fabrics, microporous, hydrophilic and bi-component

WBFs.

4.1. Waterproof breathable fabrics

4.1.1. Tightly woven WBFs

Tightly woven fabrics can be made from natural or synthetic fibres. The fabric structure is

responsible for waterproofness. For example, in the case of Ventile, large pores are avoided

by minimising crimp in the weft and using double-warp yarns [192]. The cotton fibres of

Ventile swell when wetted or exposed to high humidity, decreasing the inter-yarn pores

from 10 mm to 3 mm [193]; in this sense, Ventile can be considered a ‘smart’ fabric, since

waterproofness is improved in conditions when it is required, and air permeability and

breathability improve when waterproofness is not required [194,195]. Tightly woven fab-

rics of synthetic fibres do not display this property but still impart water resistance.

4.1.2. Microporous WBFs – traditional

Laminated or coated microporous WBFs are generally created from polymers such as poly-

tetrafluoroethylene (PTFE) or PU; the microporous structure can be created via mechanical

fibrillation (bi-axial stretching at high temperature), wet cast coagulation (exposing viscous

PU to steam) or water vapour and salt dissolution (salt crystals added and subsequently

removed to create pores) [196]. Such films or coatings feature one to two billion pores per

square centimetre ranging in size from 0.1 mm to 3 mm [197]. Because water vapour mole-

cules have diameters of 0.00004 mm, whereas liquid water droplets have diameters of at

least 100 mm, the complex passageways formed by these pores act as a liquid water filter

whilst still allowing the passage of water vapour. Because microporous coatings or films

can be compromised by water contaminated by detergents, sweat resides or salt water


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(lowering the surface tension and allowing the water to make a lower contact angle with the

porous network), many are coated with a solid hydrophilic layer [106,198] (bi-component

WBFs). A novel method of preventing this effect led to the event� membrane [199].

Protecting the PTFE membrane without a hydrophilic coating was achieved by coating the

structure with a hydrophobic and oleophobic fluoropolymer; to achieve this, a wetting

agent is required to ensure the coating has a suitably low surface tension to wet and enter

the pores of the membrane, then a heating process evaporates the wetting agent.

4.1.3. Microporous WBFs – nanofibre

Recently a new category of microporous membranes or coatings have emerged: these are

electrospun nanofibres. Garments based on electronspun nanowebs are already commer-

cially available in the form of Polartec� NeoShell�: this is manufactured by Finetex, Inc.,

using a patented procedure [200]. In one study examining a number of bespoke and com-

mercially available nanofibre-based WBFs, it was found that the Finetex, Inc., mass pro-

duced nanofibre web had the best combination of waterproofness and breathability [201].

Researchers have already conducted laboratory tests and human subject trials with micro-

porous nanoweb WBFs: Lee and Obendorf [202] laminated electrospun nanofibres and

conventional spunbond non-wovens and found that the penetration of liquid water was

reduced whilst retaining a good air permeability in comparison to currently available pro-

tective fabrics, and with no significant reduction in moisture vapour transport. This air

permeability may improve comfort [203]. Lee and Obendorf observed that electrospun

nanofibre webs have pores that are larger than those in conventional microporous mem-

branes but smaller than those in conventional spunbond non-wovens used for protective

clothing [204]. Gibson et al. [205] propose that distortion effects on elastomeric-based

nanofibre webs may significantly affect the transport behaviour. Bagherzadeh et al. [201]

laminated electrospun nanofibre webs and woven fabrics, and found that electrospun

nanofibre mats also have improved water vapour transfer properties compared to conven-

tional PTFE-based WBFs and ‘acceptable’ (i.e. lower) waterproofness. Ahn et al. [206]

also conducted human wearer trials and found that whilst the water resistance of nanofibre

webs was indeed lower, it is probably sufficient for protection from rain. In their wearer

trials, they found evidence that the nanofibre WBFs provide more comfortable conditions

in dry weather (evidenced by lower clothing microclimate absolute humidity), but in wet

weather (simulated rain) there was no difference between conventional PTFE-based

WBFs and nanofibre WBFs. Rietveld [207,208] conducted field trials comparing conven-

tional WBFs and newly commercially available nanofibre-based WBFs. The clothing

microclimate humidity was recorded, and it was concluded that all types of WBFs accu-

mulated condensation and that the new ‘air-permeable’ membranes, event� and Neo-

Shell�, represent a small but measureable advantage in terms of microclimate humidity

and drying time. Rietveld perceived ventilation to be of more importance than fabric

breathability, and that wearing a rucksack compromised the performance of all the gar-

ments on test.

4.1.4. Solid hydrophilic coating or laminated membrane

‘Hydrophilic’ WBFs are based on block co-polymers usually comprising PU or PES and

polyethylene oxide (PEO) or polyethylene glycol (PEG) (often 40%–60% weight for

weight, respectively) [106,209]. They are essentially non-porous (in the conventional

sense) and impermeable to air. Hydrophilic coatings are not monolithic but instead feature


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distinct functional layers defined as the polyethylene glycol ‘tie coat’ which lies next to

the face fabric, the ‘second coat’ which lies in the middle and finally the ‘top coat’. The

tie coat must be highly flexible and very breathable in order for desirable fabric handle

and moisture vapour transmission (MVT) through the fabric interstices. It must also

adhere sufficiently to the yarns of the fabric; these characteristics are usually obtained by

manipulating the degree of polymer cross linking and increasing the PEO quantity. The

top coat is engineered to be durable, thermoplastic for seam taping and wettable in order

to maximise absorption of water vapour. This is achieved by using lower quantities of

PEO and avoiding cross linking.

Hydrophilic films and coatings swell in conditions of high water vapour pressure,

becoming more breathable as a result. They can thus be described as smart or intelligent

textile products. At low water vapour pressure, hard PU segments agglomerate together

via inter- and intra-molecular hydrogen bonding, effectively forming a constraining mesh

around the soft PEO segment. As water vapour pressure increases, the PEO–PU mem-

brane swells, creating a more PEO-rich surface cable of increased MVT.

The process of MVT through these coatings has been described as one of ‘stepping

stones’ or ‘molecular wicking’ [210,211]. The film functions because water molecules

have a stronger affinity for the functional PEO molecules than to other water molecules

(34–38 kJ mol�1 vs. 19–23 kJ mol�1). The similarity between the adjacent O–O distance

(2.8 A�) in the PEO and water molecule clusters accounts for the high permeability of

PU–PEO films and coatings [106]. Weak hydrogen bonds are formed between the electro-

negative oxygen of the water molecules and the electropositive outer of the PEO (in some

instances PEG): this is described as a ‘hydration sheath’ [106]. Compared to covalent

bonds between oxygen and hydrogen (O–H), hydrogen bonds (H–H) are only 4% as

strong (492 kJ mol�1 vs. 23 kJ mol�1). Thus, these bonds are easily broken and water

molecules are displaced along the chain due to the water vapour pressure gradient created

by the temperature and humidity on either side of the hydrophilic layer.

4.1.5. Bi-component WBFs

Bi-component WBF fabrics are a combination of the previously described microporous

and hydrophilic fabrics, usually laminated to a tightly woven face fabric. GORE-TEX� is

the most well known example of a bi-component WBF. In this case, the hydrophilic coat-

ing partially fills the microporous structure [198].

4.2. Performance of WBFs and their differences

4.2.1. Fick’s law and WBFs

Fick first proposed the laws of diffusion based on Fourier’s laws of heat transfer [210].

Rearranged to give the dimensions of most MVT tests, Fick’s law states that the flux

of a substance Q per unit area A per time T is proportional to the concentration gradient

(c1 – c2):

Q

At¼ D

c1 � c2

l;

where D is the diffusion coefficient and l is the thickness of the fabric, membrane or coat-

ing. This equation is only appropriate for steady-state diffusion where D is constant, such

as with a microporous PTFE membrane or tightly woven synthetic fabric. Hygroscopic


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natural fibres such as cotton used in Ventile, and hydrophilic coatings, interact with the

diffusing water vapour and the diffusion coefficient D can vary.

Due to this dependency on the water vapour concentration gradient, c1–c2, the water

vapour transfer rate of 10,000 g m�2 d�1, which is required at high exercise intensities

(up to 1000 W), is unobtainable at realistic pressure gradients (100–300 Pa) [106]. There

is a large discrepancy between such real-life pressure gradients and those created in meth-

ods to test the ‘breathability’ of fabrics: 3093, 3168 and 818 Pa for ISO 11092, ISO 15946

and ISO 8096, respectively.

4.2.2. Effects of temperature on diffusion resistance

Different types of membrane are affected by low temperature and rain in different ways.

Hydrophilic membranes show greater water uptake at lower temperatures: the magnitude

of this effect is dependent on the PEO content and chain length [106]. There is some con-

flicting evidence regarding the performance of hydrophilic WBFs in cold conditions in

the literature. Osczevski found that the diffusion resistance of the hydrophilic component

of GORE-TEX� increased greatly at sub-zero temperatures such that MVT was reduced

to just a few per cent of its room temperature value [212]. Umbach and Bartels conducted

wearer trials showing that moisture accumulation was significantly lower in hydrophilic

garments than in impermeable garments, which somewhat contradicted Osczevski’s find-

ings [213]. Gibson elucidated this apparent contraction by independently controlling tem-

perature and humidity gradients, using a dynamic moisture permeation cell and showing

that the diffusion resistance of hydrophilic coatings is far more affected by humidity than

temperature, and that Osczevski’s misinterpretation was due to the inability of his appara-

tus to control humidity on either side of the sample [214]. Despite this, decreases in MVT

at low temperatures are likely due to the relationship between saturated vapour pressure

and temperature, rather than due to changes in polymer behaviour. Microporous WBFs

show little temperature dependence in terms of changes in polymer behaviour [214].

4.2.3. Effects of low temperature and rain

The main concern regarding the use of WBFs in cold or rainy conditions is the formation

of condensation on the inner surface of the WBF. Whereas microporous WBFs can

become physically blocked by rain, rendering them impermeable [192], solid hydrophilic

and bi-component WBFs continue ‘breathing’, provided there is a favourable water

vapour pressure gradient. Both Gretton [215] and Rossi et al. [216] found that condensa-

tion accumulation in rain, and at low temperatures, is largest in microporous WBFs, fol-

lowed by hydrophilic and bi-component WBFs. Gretton also found the same ranking

in field trials [194]. In simulated conditions of rain at 5�C, MVT rates of 0, 1050 and

1800 g m�2 d�1 were measured for microporous, hydrophilic and bi-component WBFs,

respectively [215]. The condensation accumulation was proportional to that measured in

wearer trials, and inversely proportional to the temperature gradient across the shell fab-

ric. The poor performance of microporous membranes is attributed either to physical

blocking of pores by rain or by the condensation of moisture within the microporous

structure itself [216]. Since the hydrophilic component of pure hydrophilic and bi-compo-

nent membranes continues to ‘breathe’, Gretton attributed the improved performance of

bi-component WBFs to the trapped air layer in the PTFE structure which maintained a

measurably larger temperature difference across the fabric, inhibiting condensation [215].

More simply, lower condensation would also be expected in fabrics with low evaporative


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resistance [217,218]. It has also been suggested that a tricot layer with good transverse

wicking properties attached to the shell inner will also promote re-evaporation through

the WBF as the condensation is spread over, and can evaporate from, a larger area.

The durable water repellent (DWR) coating of the shell layer is also extremely impor-

tant in minimising condensation accumulation [15,215,219]. With a poor DWR, the fabric

‘wets out’ and the difference in surface temperature between the inside and outside of the

shell garment quickly decreases [215,219]. The increase in conductivity related to this

increases condensation accumulation, thus causing a decrease in MVT. These effects con-

tinue in a ‘vicious circle of events’, leading to increasing condensation accumulation and

poor MVT. Whilst wind without precipitation tends to increase MVT by disturbing the

boundary layer and maintaining a favourable moisture vapour pressure gradient, wind in

combination with rain increases condensation as the shell fabric is cooled, leading to

increased condensation accumulation [220,221].


The various types of WBF offer different benefits and have their own unique drawbacks.

The poor performance of microporous membranes in terms of condensation accumulation

and limited breathability in rain has likely been addressed by the new PTFE-based event�

membrane and electrospun nanofibre web NeoShell�. In many conditions, the air perme-

ability of these WBFs may present an advantage in terms of thermal comfort, but in

very windy conditions, continuous, air-impermeable coatings such as those used in

SympaTex� and GORE-TEX� may prove advantageous in terms of conserving body

heat. The durability of bi-component WBFs may also be greater due to their ‘composite’

construction. One area where significant improvements may be made is in super hydro-

phobic coatings. Silicone nanofilament coatings may present significant advantages in

this area [222,223], and future biomimetic technology may be able to self-heal using sim-

ilar self-assembly mechanisms to those demonstrated by the original super hydrophobic

material, the lotus leaf [224,225].

5. Further discussion

5.1. The complete clothing system: interaction

So far it has been attempted to discuss the separate layers of a typical three-layer clothing

system independently. Of course, when these layers are worn together, they interact. The

simplest example of this is that when fabrics are worn as an ensemble, the total clothing

insulation is more than the sum of the thermal resistance of each material layer: Havenith

et al. [112] estimate that for a two-layer clothing ensemble, more than 60% of the total

insulation can be attributed to air layers between the skin–fabric and fabric–fabric layers,

and less than 40% can be attributed to the clothing layers themselves. This effect is also

valid for evaporative resistance. Condensation accumulation in clothing can also be modi-

fied by changing the order or fabric layers, without changing the actual fabric layers

themselves: Yoo and Kim [226] found that by placing an insulating fabric next to the

outer shell layer, rather than an air gap, condensation accumulation was reduced. This

effect may explain data suggesting that more condensation accumulates in membrane vs.

non-membrane soft-shell garments, the latter of which often feature an insulating pile or

fleece layer attached to the tightly woven shell [15]. Liquid water in the clothing system,

be it condensed sweat, non-evaporated sweat or rain, can be transferred between the


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clothing layers [227]. The amount of transfer has been shown to be partly dependent on

the pressure applied to the two fabrics in contact, which could, for example, be created by

wearing a rucksack [228]. When the pressure applied to the two fabrics is low, there is lit-

tle or no transfer of liquid water, and at higher pressures there are no voids in the fabric

layer for water to enter. Additionally, the orientation of single-sided fleece affects liquid

transfer between layers: when the pile side of the dry fabric contacted either side of the

wet fabric, there was no liquid transfer, and when the backing side of the dry fabric con-

tacted either side of the wet fabric, there was. The larger amount of liquid water transfer

observed with backing-to-backing orientation is due to the small capillary size, which

effect was also observed by Osczevski and Crow [37] who found that water uptake was

related to the pore size. In conclusion, in addition to considering the structure and proper-

ties of the individual layers, the designer of a clothing system must consider how these

layers will interact. Especially important is to try and avoid the problem of subsequent

layers attenuating the properties of the underwear and insulation: Laing et al. [79] showed

that if two base layers with measurably different properties are combined with another

mid or shell layer, it is possible for there to be no measurable difference in the new assem-

bly. Numerical thermal manikin and thermo-physiological models (e.g. [229–231]), and

computational fluid dynamics software (e.g. [232]), may prove to be increasingly useful

for designers and textile engineers attempting to predict the behaviour of complete cloth-

ing systems.

5.2. Test methods for the appraisal of clothing

Both Umbach [233] and Goldman [102] have developed similar ‘multi-layer’ systems for

the assessment of clothing system performance. At the first level, a large number of test

methods are conducted with fabric samples to ascertain the basic structural parameters

and properties of fabrics. These methods are quick, precise and logistically simple. The

second level is the first test of garments, using a thermal manikin to provide objective and

precise data. The final level tests garments using human subjects either in controlled labo-

ratory conditions or in real-use. At this level, testing becomes more logistically complex,

and explaining the behaviour of the clothing system based on fundamental principles

becomes more difficult, due to the large number of physiological, environmental and tex-

tile variables. However, this testing does provide important information about the func-

tionality of the clothing in target-use conditions. Goldman states that with each

incremental level of testing, yield of scientific information and reproducibility decrease,

and the cost and number of potentially confounding variables increase.

Most test methods used to determine characteristics such as thickness, mass per unit

area and thermal properties are used without much discussion or debate as to the validity

of the method. As discussed in Section 3.5, it is only extremely open fabrics that are

highly influenced by air flow, and allow unusually large heat transfer by radiation, that

cause discrepancies in thermal resistance determination. However, the determination of

the moisture vapour permeability of fabrics has caused substantial controversy, espe-

cially when the use of different methods has been selected to suit inflated marketing

claims [106].

The main reason that these tests have caused so much controversy lies in the simple

fact that the water vapour pressure gradients created across the WBF are far larger than

usually present in real-use. As previously mentioned in Section 4, the water vapour pres-

sure gradient in real-life is only a few hundred Pa, as opposed to a few thousand in most

test methods [106]. The inverted-cup method, BS EN ISO 15496, produces a water vapour


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pressure gradient of 2168 Pa using desiccants, and although being widely discredited in

terms of its relation to real-life, is fast and therefore a useful quality control measure. Only

when the results are applied to real-life scenarios do problems arise. The sweating-guarded

hot plate method [170] works on the principle of a textile being placed on a heated plate

saturated water; the test is isothermal (35�C), but the environment is maintained at 40%

RH and the saturated plate assumed to be 100% RH. Because of the high temperature in

the hot plate method, and low humidity next to the desiccant in the inverted-cup method,

both methods underrate hydrophilic membranes which are engineered to function opti-

mally at low temperatures, and show greater permeability in high humidity or when wet

(as with condensation in real-life). This has led to the hot plate method being described as

‘utterly meaningless for foul-weather garments worn in real-life’ [106].

5.3. ‘Smart materials’

Smart materials can be defined as ‘materials that exhibit relatively large and dramatic

physical, chemical and/or biological changes in response to external stimuli’ [234].

Smart textiles has been described as ‘one of the great catch phases of 21st century

textiles’, and due to the limited number of commercial products, it has been suggested

that the research in this area is ‘in danger of losing. . . credibility’ [235]. Though prog-

ress has perhaps been slower than the optimistic claims of some researchers and popu-

lar science publications might suggest, there is still some promise of smart materials

improving the performance and functionality of protective clothing. For example,

advances in energy harvesting from body movement and warmth continue to be made

using piezoelectric and thermoelectric textiles, respectively [236,237]. Such technol-

ogy may allow the partial powering of mobile phone or GPS devices which are popu-

lar with outdoor enthusiasts. Piezoelectric textiles can also be used to monitor vital

body signals such as respiration and pulse rate, and can act as input keys for textile

keyboards or buzzer alerts [236]. Shape memory polymers (SMPs) have also received

attention from researchers interested in textile applications. Potential applications

with SMPs range from temperature-dependent, water vapour permeable membranes to

garments that can roll up their own sleeves [238,239]. SMPs can also be used to con-

struct insulation that changes its thickness depending on the ambient temperature;

while this change in insulation is not fully sufficient to account for all changes in the

ambient temperature, it has been shown to extend the useful temperature range of an

item of clothing [240]. For outdoor activities requiring body armour, such as mountain

biking or extreme snow sports, the use of magnetostrictive materials in combination

with fabrics, allowing ‘smart’ or adjustable armour stiffness and impact dissipation, is

a possibility [241]. Chromogenic and halochromic textiles have seen applications in

industrial settings, where the colour-changing fabric may alert the wearer to hazards

such as carbon monoxide or dangerous acid vapours [242]. While applications in out-

door clothing are less clear, such chromogenic textiles might offer improved aesthetic

appeal to consumers [243]. Finally, a silicone-based, self-healing textile is now a com-

mercial reality, potentially improving the longevity of outdoor equipment [244].

6. Summary and conclusions

� The material properties and textile structures comprising an outdoor clothing sys-

tem can have an effect on human safety, comfort and performance. This literature


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review has shown that the effect of fibre type and textile structure on the wearer can

be highly dependent on the environmental conditions and type of activity

conducted.

� The layers of a clothing system interact and behave differently as a system from

that might be expected from their individual properties. Therefore, designers of

clothing systems face a considerable challenge to ensure not only that the most

appropriate materials and structure are selected for each layer, but that that as a sys-

tem, these layers interact desirably: i.e. they do not attenuate or nullify desired

properties of the other layers.

� Test methods must be carefully considered to avoid producing misleading

information.

� Many other factors which have not been addressed in this review must be consid-

ered, such as clothing fit and ergonomic factors, clothing weight [245], design and

styling. Only by considering all these factors can clothing for the outdoors be opti-

mised, ensuring widespread, safe and comfortable participation in outdoor activities.

Acknowledgements

The first author would like to acknowledge Mr Dave Brook at the University of Leeds for his bril-liant lectures which were instrumental in inspiring the author to undertake research in technical tex-tiles. Credit is also due to Dr Mark Taylor, also at the University of Leeds, for years of advice,discussion and debate about this topic. Thanks also to Dr Robert Lomax, Dr Ren�e Rossi and theEditor-In-Chief Professor Richard Murray for encouraging and enabling the author to write andpublish this literature review.

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