8/6/2019 Stw Research 0

http://slidepdf.com/reader/full/stw-research-0 1/38 - AI 223

R O D U C T S & T E C H N O L O G Y

D O U B L E S K I N F A ç A D E S

ouble Skin Faades:

esign and Technology for the Irish Climate

is is the frst in a series o technical articles promoting a better 

derstanding o sustainable and low-energy design solutions, with

rticular ocus on their appropriateness to the Irish climate.

ouble skin açades have long been a eature o architectural

mpetitions and easibility studies, however, until recently,

w designs in Ireland were realised. This was due to a

umber o actors, including unavourable cost reporting

payback periods; an un-readiness to reduce plant size

d associated costs due to lack o perormance certainty ;

client scepticism. Within the last 12 months, a number

buildings, particular in Dublin, are proudly displaying

eir new ‘double skin’, and there are as many on the

awing boards. What has driven this change? Have market

rces demanded a higher perormance açade, or is this

consequence o the new Part L Regulations, or a greater

esire to develop low energy solutions?

e development o the double skin açade has been

omoted as the answer to providing a ully glazed curtain

all and as an eective way o controlling heat, light, air and

oise through the building envelope as well as reducingnergy consumption. These claims depend, however, on

e particular climate, site, and orientation that the double

in is placed. So what are the main attributes o double

in açades in relation to the Ir ish climate, and what

ngurations are most suitable?

eduction of Solar Heat Gain

e primary attribute o a double açade is its ability

reduce solar heat gain. Allowing or occupancy and

quipment, the solar gain to a building is by ar the largest

eat element, which must be reduced in order that it may

e naturally ventilated or utilise low energy techniques to

ovide a comortable working environment.

e simplest way to reduce solar heat gain to a building is

reduce the surace area o the glazing, shade the glazing

use body tints / ritting to improve the solar perormance

the glazing. An oce building in Ireland will typically be

ccessully naturally ventilated i the solar heat gain to the

oor is limited to the region o 15 – 25 W/m2. In the case o a

est açade with low-E double glazing, this would equate to a

aximum percentage glazing o 25-40%. For many building

pes such as open plan o ce spaces, this may prove too

strictive.

here large surace areas o glazing are preerred, solar heat

in can be reduced by providing shading or solar coating to

e glazing. External shading is potentially the most eective

means o reducing solar gain, as all heat is removed prior

to reaching the building. However, by their nature, these

shading systems will oten compromise views and have

more complex maintenance issues. This is particularly true o 

south açades, where a horizontal brise soleil is eective in

removing solar heat gain in summer-time when the sun is at

a relatively high altitude (50-60o).

However, horizontal external shading alone is not eective

to east/west açades in Ireland, due to low sun angles in

summer. The solar altitude at 10:00/ 16:00 in August would

only provide 500mm shading or a 1-m deep brise soleil or

east/west açades respectively. West açades are particularly

problematic, as they have to be shaded during the hottest

time o the day, the aternoon.

One option to achieve reduction in solar gain is to improve

the solar properties o the glazing itsel, such as tinted

or refective glazing. While solar coatings have improved

in recent years, there will be a relative decrease in light

transmission and daylighting quality and physiologically can

create a sense o separation rom the ex terior with a relianceon articial lighting to create a comortable environment.

Body tints will not necessarily deal with glare and thereore

internal blinds are oten used, which urther reduce daylighting.

Double skin açades are thereore particularly suited in

Ireland to west (and east) açades, where horizontal shading

such as brise soleil cannot be eectively implemented. The

benet o the double skin açade is that a vertical shield,

such as Venetian blinds, eective or all solar angles, is

provided. Furthermore, the blinds can be raised during

cloudy conditions, which is particularly important in terms o 

maximising daylight or Ireland’s variable climate. As the blind

is contained within the double açade cavity, it is not exposed

to the elements, and vastly easier to maintain and clean thanexternal shading elements.

Ventilation

Ireland is particularly suited to naturally ventilated buildings,

as external air temperatures rarely exceed design internal

temperatures [see Figure 2]. The advantage o the two layers

o glazing allows controllable natural ventilation without the

gusts and breezes associated with openable windows. This is

particularly true or buildings at an exposed site or or taller

urban buildings where wind pressures increase at higher

storeys. The extended air path into the building also controls

issues such as noise inltration, dust and insect inltration.

I the açade is to be used to naturally ventilate the internal

space, it will typically consist o ex ternal louvres and dampers,

by Andrew Morrison and David Walshe

1

8/6/2019 Stw Research 0

http://slidepdf.com/reader/full/stw-research-0 2/3

8/6/2019 Stw Research 0

http://slidepdf.com/reader/full/stw-research-0 3/30 - AI 223

Full Height Ventilated Flue vs. Single Storey

entilated Faade

e advantages o the single-storey ventilated açade are that

s generally a simpler solution in terms o construction, re

paration, and noise transer between foors. A single storey

çade is more suitable or naturally ventilated buildings as

esh, cool air is available or occupants at each foor level.

contrast, a ull height ventilated fue would have solar

eat build up over a number o foors, which may mean that

mperatures at the top o the fue are too excessive to be

ed or natural ventilation to the occupied space.

s essential that ull height fues utilised in naturally

ntilated buildings are used solely or exhaust air, cross

ntilating rom opening windows on other açades. In this

nguration, the fue is acting in a similar manner to an

rium and the exhaust louvres normally required to extend

above the roo level in order to ensure that hot air does not-circulate to the highest foor level. The principle o this is

milar to ensuring a smoke layer in an atrium. The ull height

ue does, however, have the aesthetic advantage o not

quiring louvres at each foor level.

ptimum double skin açade fue widths are 200 to 400mm

r single-storey fues, which require all sections o the

ternal skin to be openable or maintenance. Full height

ntilated fues are o the order o 80 0 to 1200mm, and an

cess platorm or maintenance can be provided, with less

pening required to the inner skin, and less disruption to the

ternal space, during maintenance.

External Skin - Double Glazed or Single Glazed

sing a single glazed external skin can create a very

ansparent external appearance, particularly when using

w iron glazing. Blinds and solar shading devices (preerably

ntrollable) can then be placed within the cavity and the

lar gain on the blinds can be removed by ventilating the

vity. Openable windows into the fue as a means o providing

tural ventilation must be careully considered, as there is a

k o warm air rom the occupied areas causing condensation

n the cold external glazed skin. By placing the double glazed

nits on the external ace, condensation can be avoided.

ouble glazed units are best suited to be positioned to the

ternal skin or naturally ventilated solutions, whereas

ybrid / mechanical schemes can be achieved with a single

azed external skin.

3. Double Skin Faades with Air Conditioning/ Mechanical

Ventilation vs Naturally Ventilated Systems

 The double skin açade can also be used in conjunction with

an air conditioned or mechanically ventilated building. In

this instance, the primary purpose o the double açade is to

reduce solar heat gains and subsequently air conditioning

cooling loads. The external fue can be used either as a

stand-alone system or removing solar heat gains by natural

convection (with the building sealed rom the açade) or used

as a return air path or the mechanical ventilation network,

which has the advantage o the heat trapped in the double

skin fue being available or heat reclaim at central plant.

While the above mechanically ventilated/ hybrid

congurations are most popular world-wide, it is also oten

possible in most building applications in Ireland to utilise

a double skin açade with an entirely naturally ventilated

building, due to the moderate climate.

 The benets o a ull naturally ventilated system are energy

consumption, increased uel costs, and the cost saving o 

mechanical plant, as well as carbon emissions, and low

energy credentials. There are also comort issues as many

people preer naturally ventilated spaces, provided the

ventilation is controllable and overheating is not a problem.

Another option is to provide a mixed mode system. This has

two meanings; either that the building is partially naturally

ventilated/partially air conditioned, or that the building

operates with natural ventilation in spring/autumn, and with

air conditioning in summer.

However, regardless o preerred system, it must be noted

that in order to achieve successul operation, occupant users

need to play a role in operating and maintaining the systems,

and their eectiveness, will oten depend on a combination

o user control and a sophisticated Building Control System.

Note:

Graphs courtesy o IN2 Engineering Design Partnership.

 Acoustic data courtesy o AWN Consulting Engineering.

Cost data courtesy o KSN.

Recent examples o double skin

açades in ofce buildings in Dublin

Riverside One,

Scott Tallon Walker Architects

Riverside Two,KMD Architecture

Photos by Dennis Gilbert 

1-6

1,2,4,5

3,6

3 4 5 6