how_to_airtightness

8/7/2019 How_to_Airtightness

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Airtightness and air leakageAir leakageAir leakage is the uncontrolled exchange of air both into (infiltration) and

out of (exfiltration) a building through cracks, gaps and other unintentional

openings in the building envelope. The rate of air leakage depends upon the

air permeability of the construction, the wind speed and direction, and the

temperature difference between the inside and outside of the building, as wellas within the building.

AirtightnessAirtightness is the measurement criteria used to evaluate the air leakage of a

building. It determines the uncontrolled background ventilation or leakage rate

of a building which, together with purpose-provided ventilation, makes up the

total ventilation rate for the building. Traditionally, air leakage was expressed

in air changes per hour (ach or h-1), however currently air permeability is used

(m3/(h.m2) as it takes into consideration the effects of shape and size. The

more airtight a building, the lower the air permeability.

In the UK, airtightness is measured at an artificially induced pressure of 50Pa

(n50).

Measuring airtightnessThe airtightness of a building envelope can be measured using the fan

pressurisation (blower door) technique or the tracer gas technique. The fan

pressurisation technique is the simplest, quickest and most widely used (see

Figure 1). It involves sealing a portable variable speed fan into an external

doorway using an adjustable door frame and panel. A fan speed controller

is then used to pressurise and/or depressurise the building. The airflow rate

that is required to maintain a number of particular pressure differences across

the building envelope is then measured and recorded on a pressure and flow

gauge. The leakier the building, the greater the air flow required to maintain a

given pressure d ifferential.

How to achieve good levelsof airtightness in masonry homes

Dr David Johnston & Dominic Miles-Shenton

IntroductionIn the UK, as in most industrialised countries,

the domestic sector contributes substantially to

national energy use and CO2 emissions. Currently,

there are over 25 million dwellings in the UK

accounting for just under 30% of the UK’s total

CO2 emissions. This is a substantial figure given thatthe UK housing stock is categorised by long physical

lifetimes and slow stock turnover. Therefore, if we

are to mitigate against climate change and achieve

the Government’s target of an 80% reduction in

national CO2 emissions by 2050 based on 1990

levels, then significant reductions in the carbon

emissions from dwellings both new and existing will

be required.

One factor that can have significant impact on

the energy use and CO2 emissions attributable todwellings is airtightness. The current regulatory

requirement for airtightness is a design air

permeability of 10 m3/(h.m2) @ 50Pa. This is

linked to the target carbon dioxide emission

rate (TER1). However, in order to cost effectively

achieve a satisfactory carbon emission rate or a

particular level within the Code for Sustainable

Homes, designers are likely to make trade-offs

between fabric and system performance, and an

air permeability of 5 m3/(h.m2) or less is likely to

become a more typical design requirement.

This guide gives an introduction to the topics

of airtightness and air leakage and discusses the

basic principles of airtightness. It also illustrates a

number of areas within masonry construction that

may contribute to air leakage and identifies ways in

which air permeability of less than 5 m3/(h.m2) @

50Pa could be consistently achieved in typical UK

volume housing.

1 The TER is the minimum energy performance requirement for new dwellings approved by theSecretary for State. It is expressed in terms of kgCO2/m2 per annum emitted as a result of theprovision of heating, hot water, ventilation and internal fixed lighting for a standardised householdwhen assessed using approved calculation tools.



2

Figure 1Measuring airtightness using the fan pressurisation technique.

Identifying air leakageThe most widely-used technique for identifying the main areas of air

leakage within a domestic building is smoke detection. This technique

involves either pressurising or depressurising the building, and then

locating the areas of air leakage using a manual or electronically

operated hand-held smoke puffer. In the majority of cases, the

dwelling is pressurised and leakage detection is performed from

within the dwelling, as it is much easier to identify where the smoke

leaks out of the habitable space, rather than into the habitable space.

However, it is important to realise that in most cases, the smoke

puffers are only able to identify the point where the smoke leaks out

of the habitable space, rather than the path that the smoke takesfrom the inside to the outside of the building.

Infrared thermal imaging can also be used to identify areas of air

leakage. Although it can provide additional information which is

not always possible to recognise purely by smoke detection, it is

considerably more complex and problematic. There are also limitations

as to when and where it can be used as a detection technique.

Direct and indirect air leakageAir will leak through porous building materials and unintentional

cracks, gaps and openings in the building envelope. This leakage canoccur directly and indirectly. Direct air leakage points are points in the

envelope where the air leakage occurs directly through the primary

air barrier from inside the insulated envelope to outside or vice versa.

Indirect air leakage points are points in the envelope where the air

leakage occurs indirectly through the primary air barrier via a series of

interconnected voids from inside the insulated envelope to outside or

vice versa. Common examples of direct and indirect air leakage points

are given in Table 1.

Experience indicates that the majority of air leakage within UK

dwellings occurs through indirect rather than direct leakage points.

These indirect air leakage paths are often complicated, making it very

difficult, if not impossible, to trace and seal them effectively.

Table 1Common examples of direct and indirect air leakage points.

Direct leakage points Indirect leakage points

Around trickle ventilators andthrough poorly- closing trickle

ventilators.

On external and party walls at theground floor/skirting board junction.

Around and through the loft hatch. Under kitchen and utility room units.

Through gaps at bay windows. Around staircases.

Around poorly fitting windows anddoors.

Into intermediate floor voids.

Around sliding mechanism of patiodoors.

Into service voids (e.g. behind bathpanels)

At thresholds. At intermediate floor perimeters..

Around services at the point wherethey penetrate through the primaryair barrier.

At service penetrations where theypenetrate the dry-lining and/orinternal finish.

Therefore, it is much more effective and a much more robust

approach to design and construct airtight dwellings in the first

instance, rather than to try and carry out post construction remedial

airtightness work by for instance plugging gaps in surface defects

(secondary sealing) once the dwelling has been built.

Airtightness and ventilationBuildings are ventilated via a combination of purpose-provided

ventilation and infiltration. Purpose-provided ventilation is the

controllable air exchange between the inside and outside of a building

by means of a range of natural and/or mechanical devices. Infiltration

is the uncontrollable air exchange between the inside and outside of

a building through a wide range of air leakage paths in the buildingstructure.

The level of airtightness achieved within a building will have an

important influence on the ventilation rates that can be achieved

and the type of ventilation strategy that should be adopted. Careful

consideration should be given to the ventilation strategy, particularly

if a design air permeability target of 5 m3/(h.m2) @ 50Pa or less

is adopted. However, irrespective of whatever ventilation strategy

is adopted, the aim should always be to minimise the amount of

uncontrolled and usually unwanted infiltration by making the building

envelope as airtight as possible, and then ventilate the buildingappropriately by providing sufficient purpose-provided ventilation. In

other words, ‘build tight, ventilate right’ . It should be remembered that

a dwelling cannot be too airtight, but it can be under ventilated.

Methods of achieving sufficient purpose-provided ventilation are

currently contained within the Building Regulations 2000 Approved

Document Part F 2010 edition [8]. This includes separate guidance

for those dwellings that are designed to have an air permeability

greater than 5 m3/(h.m2) @ 50Pa and an enhanced provision for those

dwellings designed to have an air permeability less than or equal to 5

m3/(h.m2) @ 50Pa.



3

How to achieve good levels of airtightness in masonry homes

Airtightness RequirementsApproved Document Part L1A 2010

Airtightness is currently addressed in Approved Document PartL1A 2010 (ADL1A 2010) of the Building Regulations [9]. ADL1A

2010 requires that the building fabric should be constructed to a

reasonable quality of construction so that the air permeability is

within reasonable limits. Guidance on a reasonable limit for the

design air permeability2 is given as 10 m3/(h.m2) @ 50Pa. In the

majority of cases, complying with the regulation will require some

degree of compulsory pressure testing3. Details of the testing regime

associated with each method of compliance can be found within

ADL1A 2010 (see [9]). For dwellings that have not been pressured

tested, the assessed air permeability is the average test result obtained

for dwellings of the same type which have been tested plus a margin

of 2 m3/(h.m2), to account for the likely variability of air leakage that

would be achieved by on-site testing. The outcome of this change

to the Regulations is that the design air permeability rate should not

be more than 8 m3/(h.m2), so that untested dwellings achieve an

assessed air permeability no greater than the backstop value of 10

m3/(h.m2). As a consequence, designs reliant on low air permeability

should ideally be pressure tested to avoid this performance penalty.

Compliance with ADL1A 2010 also requires that the pressure tests

are undertaken in accordance with the procedure set out in the Air

Tightness Testing and Measurement Association (ATTMA) Technical

Standard 1 – Measuring Air Permeability of Building Envelopes [1] .

Airtightness and the energyperformance of dwellings

Airtightness is crucial to improving the energy performance of

dwellings. Currently, in the UK, the temperature of the outside air

is nearly always lower than the temperature of the air inside the

building, thus, any air leakage from the inside to the outside of the

building is likely to result in:

■ A significant reduction in the effectiveness of the thermal

insulation, due to air leakage past the insulation (thermal

bypassing), resulting in increased heat loss .

■ An increase in the building’s ventilation and fabric heat losses,

resulting in an increase in space heating requirement.

■ Increased energy usage and higher carbon emissions.

2 Design air permeability is defined in ADL1A 2006 as the value of air permeability that isselected by the designer for use in the calculation of the DER.

3 Details of the pressure testing requirements are contained within Regulation 20B of TheBuilding Act 1984 (ODPM, 2006c).

It is also important to realise that in the past, when dwellings were

relatively poorly insulated, airtightness had comparatively little

influence on the overall energy performance of the building. However,

as dwellings have become better insulated, the relative importance

of airtightness has increased. In very well insulated dwellings, the

proportional effect that airtightness has on the performance of adwelling can be significant.

For example, in a notional semi-detached dwelling with a total heat

loss of just under 140W/K (roughly equivalent to a 2006 Part L1A

compliant dwelling) and an air permeability of 10 m3/(h.m2) @ 50

Pa, the ventilation heat loss is likely to account for around a third of

the dwelling’s total heat loss (see Figure 2). If no additional measures

are taken to reduce fabric heat loss beyond this level, and measures

are taken to reduce air permeability, then significant reductions

in ventilation heat loss are possible. If the air permeability of the

dwelling is reduced to 1 m

3

/(h.m

2

) @ 50 Pa and an mechanicalventilation and heat recovery (MVHR) system is installed, then

the ventilation heat loss could be reduced to just over 20W/K,

representing approximately one fifth of the dwellings total heat loss.

Figure 2Comparison of fabric and ventilation heat losses for a ‘notional’(80m2) semi-detached house.

160

140

120

100

80

60

40

20

0

10 (naturalventilation)

Air permeability (m3/(h.m2) @50Pa) and ventilation strategy adopted

Fabric Ventilation

H e a t l o s s

( W / K )




2 (mechanicalextract

ventilation)

1 (mechanicalventilation withheat recovery)

As Building Regulations become more stringent, achieving the desired

CO2 Target Emission Rate (TER), is likely to require designers to

take a more holistic approach in which reductions in the design air

permeability are coupled with investments or trade offs in additional

thermal insulation, alternative heating systems or renewable

technologies. If this is the case, an air permeability of less than 5 m3/

(h.m2) @ 50Pa is likely to become a common design requirement. For

instance, the fabric energy efficiency standard for zero carbon homes

published in 2009 suggests an air leakage target no greater than 3

m3/(h.m2) @ 50Pa, [7]. At such levels of air leakage, factors such as

the ventilation strategy chosen, heat exchanger efficiency and specific

fan power all become important and can have a significant impact on

the CO2 emissions achieved.



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Measured airtightness of new UK housingThere is a limited amount of published data available on the air

leakage of dwellings that have been built to comply with Part L1A

2006. Measurements undertaken by the NHBC on 1293 dwellings of

different size, type, and construction [7] indicate that although the

majority of the dwellings (>95%) achieved an air permeability belowthe regulatory standard of 10 m3/(h.m2) @ 50Pa first time, a wide

range of airtightness was still observed (see Figure 3). The mean of

the sample was approximately 6 m3/(h.m2) @ 50Pa. The results are

broadly consistent with those obtained by Building Sciences Limited

[2] on a separate group of 750 dwellings.

Figure 3Air permeability of 1293 dwellings built to ADL1A 2006.After NHBC [7].

300

250

200

150

100

500

0

0 - 1

1 - 2

2 - 3

3 - 4

4 - 5

5 - 6

6 - 7

7 - 8

8 - 9

9 - 1 0

1 0 - 1 1

1 1 - 1 2

1 2 - 1 3

1 3 - 1 4

1 4 - 1 5

1 5 - 1 6

1 6 - 1 7

Air permeability (m3/h.m2) @ 50Pa)

N o . o f d w e l l i n g s

A more detailed analysis of the results obtained for masonry dwellings

within both datasets suggests that, on average, dwellings with a wet

plastered finish are more airtight than dry-lined dwellings.

Principles of airtightnessHigh levels of airtightness are only likely to be achieved by

understanding and adopting a number of basic principles throughout

the design, procurement and construction of the dwelling. These

principles are as follows.

Design stageDefining a continuous and robust primary air barrier is crucial at the

design stage. This can be achieved by:

■ Identifying a line through the building that will act as the main barrier

to air leakage. This is known as the dwelling’s ‘primary air barrier’.

■ Ensure that the primary air barrier is continuous around the

thermal envelope and, where possible, in contact with the

insulation layer. This will not only minimise air leakage but also the

possibility of thermal bypass4.

■ Check the continuity of the primary air barrier by undertaking a

‘pen-on-section’ test (see Figure 4). This involves using a line to

mark the location of the primary air barrier on a set of General

Arrangement drawings. The line should be continuous and

separate the heated (conditioned) spaces from the unheated

(unconditioned) spaces.

■ From the ‘pen-on-section’ test, identify areas where additional

detailing will be required and identify those trades that are

responsible for the design and construction of the air barrier.

■ Produce large scale drawings (1:5) of any areas of complexity or

changes in plane identified by the ‘pen-on-section’ test and identify

how continuity of the primary air barrier will be maintained at

these areas.

■ Minimise the number of service penetrations through the primary

air barrier. Consider the adoption of service zones or voids that

may group services together.

■ Try and make the primary air barrier as simple as possible. Try and

avoid or at least minimise changes of plane and complex detailing.

■ Consider the impact that materials with different tolerances may

have on the primary air barrier. Ensure that any issues are resolved

at the design stage prior to commencing construction.

■

Ensure that the primary air barrier is robust, impermeable anddurable.

■ Do not be over reliant on secondary sealing, for example using

sealant to seal the junction between intermediate floors and the

skirting board, to provide part of the primary air barrier.

4 Further information on thermal bypassing can be obtained from references [12] and [13].



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Figure 4Example of a pen-on-section test undertaken on a set of GA drawings.

BATH

HALL

L.ROOM

AC STAIRS BED.3

It is important to realise that approaches that rely on high levels of

workmanship coupled with secondary sealing are likely to be lessrobust than those that rely on the identification and execution of a

continuous primary air barrier.

Sequencing of construction processesGive explicit consideration to sequencing during the design,

procurement and construction by:

■ Attempting to install the primary air barrier over as large an area

as possible in one single operation. For example, installation of

the top floor ceiling prior to the erection of the internal partitions

minimises the number of penetrations through the ceiling.

■ Ensuring that the primary air barrier can be completed, inspected,tested and repaired prior to any part of it being covered up by

other materials or finishes. For example, where a parging coat

forms the primary wall air barrier, it should be applied to walls

before any subsequent trades commence.

■ Sleeve and seal service penetrations through the primary air barrier

during installation wherever possible, to avoid the need to break

out subsequent new construction.

■ Ensuring that the method of sealing service penetrations through

the primary air barrier is robust enough to enable later fitting-out

work to take place without compromising the installed seal. For

example, electricity cables that penetrate the primary air barriershould be fitted with an appropriate seal that allows for the cables

to be manipulated during and after the installation of the terminal

fitting without detriment to the seal.

Site supervision and workmanshipEnsure that there are high standards of site supervision and

workmanship on-site. This can be achieved by:

■ Providing airtightness training as an integral part of site induction.

Both generic and trade-specific airtightness training should be

provided to all operatives on-site. Training should explain why

airtightness is important, how it is being tested, what quality controlprocesses are in place and what happens when things go wrong.

■ Ensuring that operatives know what they are required to achieve

and what constitutes an acceptable standard. The definition and

visibility of the air barrier is crucial.

Quality control

Testing, monitoring, and feedback are essential to any quality controlprocess. Specific ways in which process can be improved include:

■ Formally describing the quality control process and clearly setting

out the different roles and responsibilities for reporting, recording,

investigation and action.

■ At key stages of the construction, check the integrity of the

primary air barrier and undertake airtightness measurements

before the construction progresses to a stage where it becomes

impossible to efficiently undertake remedial action.

■ Maintain a photographic record of observations made during

the construction process. This not only allows a more precise

retrospective analysis in the event of future investigations, butalso provides useful material for training and improving awareness

among site staff of the impact of their actions.

■ As far as possible, construction specifications should ensure

standardisation of detailing to enable site teams to become

familiar with the materials, components and tolerancing needs.

Where modifications are required these should be undertaken in a

controlled way accompanied by effective detailed documentation.

CommunicationCommunication of detailed design information and feedback on

airtightness performance is crucial if high standards of airtightness areto be achieved. Effective communication requires:

■ Design information to be provided to all subcontractors and trades

that may have an impact on the integrity of the primary air barrier,

through an appropriate mixture of documentation and briefings.

The design information should include procedural specifications as

well as drawings depicting the final form. In particular, all drawings

and specifications should define the primary air barrier and detail

drawings should show how the air barrier is to be maintained at

junctions and penetrations.

■ Any modifications or deviations from the design made on site

(including ad-hoc design alterations, product substitutions and

procedural changes) should be fed back to the designers to bereassessed where necessary – particularly where there may be

implications for the air barrier integrity, thermal performance or

condensation risk.

Potential defects associated with theconstruction of the primary air barrier

■ The type and formation of the primary air barrier has a critical

influence on the airtightness achieved. Observations obtained

from masonry dwellings during their construction have highlighted

a number of recurrent areas that may contribute or lead to air

leakage.

Red lineindicateslocation of primary air barrier



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Design OptionsPlasterboard dry liningPlasterboard dry-lining is currently a common source of air leakage in

new masonry dwellings. Experience suggests that when plasterboardon dabs is used as the primary air barrier for external and separating

walls, it can be difficult to achieve an airtight seal around the edges.

The adhesive dabs that are used to seal the perimeter of the boards

are often discontinuous (see Figure 5) allowing the air behind the

plasterboard to move around freely and link with various other leakage

paths within the dwelling.

Even when considerable time and effort is spent on ensuring that

continuous ribbons of adhesive are applied around the perimeter

of the boards, these are very rarely completely solid. In addition, a

continuous channel for air movement is left around the perimeter

of the wall between the ribbons and the perimeter junctions withadjacent walls, the ceiling and the floor.

Figure 5Discontinuous ribbons of plasterboard adhesive on an external wall.

If plasterboard dry lining is used to form part of the primary air barrier,

it is likely to result in long-term airtightness performance issues as a

result of drying, shrinkage and settlement.

Parging layer and wet plastered finishAn alternative and potentially more robust solution to using

plasterboard dry-lining as the primary air barrier, is to apply a sand

and cement or gypsum based plaster parging layer to the internal face

of all of the external walls prior to the application of the dry-lining,

Alternatively, a wet plaster finish can be applied as an alternative todry lining.

A typical internal render mix is cement:sand:lime 1:4:1/2. with typical

thickness for a parge coat between 3mm and 6mm. A wet plaster

mix will be thicker, with two or three coats and will take longer to dry

out. However, it does offer the additional benefit of increased thermal

mass, which has the potential to reduce operational CO2 emissions

and enhance summertime comfort. Typical plastering or parging mixes

are given in BS EN 13914-2:2005, and additionally there is a wide

range of proprietary bagged plaster mixes, some of which are designed

to be spray applied.

The advantage of both of these techniques is that they have the

potential to seal any badly pointed joints or shrinkage cracks in the

inner leaf of masonry preventing air moving through the masonry

walls. In addition, from a quality control perspective, it is relatively

easy to see where the layer has been applied, making inspection easier.

However, care should be taken to ensure that the parging layer or wet

plastered finish is continuous and links with the air barrier in the floor,

ceiling and around openings. It is not uncommon to find areas where

this layer is not continuous (see Figure 6), particularly if the layer has

been applied after installation of elements such as intermediate floors,

partitions, stairs or services. Discontinuities can also occur at the

interface between the walls and the intermediate floor where the floor

deck, strutting and joists have been fitted prior to the application of

the parging layer (see Figure 7)

Figure 6Incomplete parging layer behind services and stairs.



7


A number of proprietary products are available, such as joist seals,

joist caps and joist ends, that are intended to provide an airtight

seal between built-in joists and the inner leaf of the masonry wall

by limiting the effects of shrinkage at the joist and the surrounding

mortar joints. However, instead of addressing the problems associated

with built-in joists, what these products do is to effectively move theproblem somewhere else. For instance, the problem of how to provide

an airtight seal is moved to the junction between the joist seal and

the mortar and the junction between the joist seal gasket and the

inner leaf of the masonry wall.

Difficulties can also arise where the joists run parallel with the walls

(see Figure 9). In such situations, the joists are slightly offset from the

inner leaf of the external or party wall to allow electrical cables to be

run from one floor to the next. The offset is typically so small that

it is not possible to apply mortar, mastic or a parge coat to the area

between the joist and the wall to seal this junction.

Figure 9Offset joist running parallel with the parged wall.

A number of alternative approaches to using built-in joists are

available that are potentially more robust. These include the use of

joist hangers, the use of continuous corbelling to support intermediate

floors or the use of an alternative floor construction such as hollow

core concrete planks or a beam and block system.

Sealing around openingsExperience suggests that it is also difficult to prevent air leakage

around openings. It is common for air movement to be observedaround window sills, either directly through small cracks and gaps

to outside or indirectly into the void behind the plasterboard. For

example, Figure 10 shows the installation of a dry-fitted sill board

with a visible gap between the sill board and the external wall (in

this case the wall is also parged). If the wall is dry lined, this gap is

likely to remain, allowing air to move freely into the gap behind the

plasterboard dry lining.

Figure 7Discontinuity in parging layer behind floor joist.

Sealing around built-in joistsAchieving an airtight seal around built-in intermediate floor joists

on masonry cavity construction tends to be difficult, partly due to

movement and shrinkage but also due to the construction processes

that are adopted when building-in the joists. Restricted access to the

junction between the blockwork and the built-in joists, particularly

where the joists run parallel to the external and/or separating walls,

make it difficult to ensure adequate sealing.

For example, where a substantial height of brickwork in the outer

leaf is constructed before the inner leaf of blockwork above theintermediate floor, it is difficult for the bricklayers to achieve full

perpends and bedding layers around and between the built-in

joists and strike off any excess mortar, as the blockwork has to be

constructed from inside the dwelling.. Any excess mortar has to be

chipped away leaving uneven surfaces that are difficult to seal with

mastic around the joists. The gaps at perpends are also difficult to fill

because of their proximity to the floor decking (see Figure 8).

Figure 8Excess mortar around built-in joists and gaps in perpends prior to dry-lining.



8

Figure 10Daylight visible through gaps beneath a window sill and subsequentair leakage at the finished detail.

Thresholds are also a common source of air leakage. Typically, air

movement occurs at the frame/wall junction, under the thresholditself and at the junction between the skirting board, the floor and the

doorframe (see Figure 11).

Figure 11Air leakage at threshold at the junction between the skirting boardand the door frame.

Air leakage around window sills and at thresholds can be minimised

by ensuring that the air barrier is continuous and connects with the

window sill and doorframe.

Through componentsComponents that penetrate the air barrier such as windows, doors,

rooflights, loft hatches and recessed lighting can be a significant

source of air leakage resulting in a direct path from the inside to

the outside of the dwelling. For example, trickle vents are often of a

poor fit or do not close properly allowing air movement through the

vent itself when closed, or through the gap between the vent and

the window (see Figure 12). Also the seals on window casements,

rooflights and loft hatches are not always fully compressed when

closed allowing air movement (see Figure 13). Leakage has also

been observed around the sliding movement of patio doors, at the junction of French doors, and in the most severe cases, gaps have

been observed between the external door and the surrounding frame

(see Figure 14). Air leakage has also been observed through recessed

lighting installed in the top floor ceiling (see Figure 15).

Figure 12

Leakage through poorly fitting trickle vent.

Figure 13Leakage through poorly sealed loft hatch.

Figure 14Observable gap between external door and frame.



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Figure 15Leakage through recessed lighting on top floor ceiling.

A number of these issues can be addressed using appropriately

specified components that are designed to be airtight.

Internal partition/ceiling junctionIt is common practice to erect the top floor partition walls before

installing the plasterboard ceiling. This practice creates numerous

potential air leakage paths into the loft space. These leaks can occur

through penetrations and gaps in the studwork, at the junctions

between the studwork and top floor ceiling, and particularly at

junctions between partition walls.

An example of the problems that this can create and that remain evenafter the installation of the plasterboard ceiling is illustrated in Figure

16. An effective way of avoiding this problem in housing is to erect

the ceiling before the partition, as shown in Figure 17. However, if the

top floor is to be dry-lined, potential air movement around the ceiling

perimeter will still need to be addressed, to ensure continuity between

the wall and ceiling air barrier. In the case of apartments, it may not

be practicable to install the ceiling prior to the partitions for fire safety

reasons.

Figure 16Partitions-first sequence of top-floor ceiling construction, from below

and above, showing potential air leakage paths.

Figure 17Alternative sequencing of construction of partitions and ceiling, withcomplete ceilings installed prior to partitions and services.

Service penetrationsSignificant air movement has been observed around unsealed or

inadequately sealed service penetrations, at the point where the

service penetration punctures the air barrier, which provides a direct

leakage path to unconditioned zones under the ground floor, in the

external cavity or into the roof space.

Penetrations through the ground f loor are often inadequately sealed.

This can be due to them occurring in hidden areas, as a result of

access restrictions or from an unsuitable choice of sealing material.

Figure 18 provides examples of air movement around a soil pipe

positioned too close to an internal block wall to seal to an acceptable

standard, and penetrations around water mains relying on permeable

materials (compacted mineral wool) to prevent air movement.

Figure 18Leakage at ground floor penetrations.

Figure 19 shows examples of electrical, ventilation and plumbing

penetrations permitting air movement between the conditioned

space and the external wall cavity. Although all these penetrations

are sealed at the external leaf to prevent water ingress, gaps behind

the electrical consumer unit, cooker-hood extract and wash basin

waste pipe have not been sealed where the services break through the

primary air barrier, the inner leaf blockwork. Such infiltration paths are

commonplace in wet areas where the build sequence results in kitchen

units, boxing-in or sanitary ware being installed before the penetration

has been sealed to an airtight standard.



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Figure 21Direct leakage paths between the soil stack and ventilated loft space.

Internal soil stacks invariably provide links for air movement around

the dwelling by connecting voids in bath or shower rooms with other

service voids, intermediate floor voids and the plenum behind the

dry-lining, allowing an escape route where the soil pipes penetrate

the primary air barrier. The linking of these voids can result in very

convoluted leakage paths, where the point at which air leakage from

the dwelling is detected may be far removed from where the actual

air barrier is punctured or discontinuous.

Figure 22 shows how similar problems can occur with external soil

pipes, in this case the pipe has not been sealed at the air barrier (the

inner leaf blockwork) prior to the tiled finish being applied. Although

the soil pipe has been sealed to the tiles in the final completed

photograph, air movement between the external cavity and the void

behind the dry-lining is unrestricted.

Figure 22Service penetration through an external wall, linking the void behindthe plasterboard to the external cavity.

A recurring problem is that service penetrations that are subsequently

hidden behind boxing or panels (for example the bath panel, shower

tray, shower pod, in an under sink unit, in an airing cupboard or

in an under stairs cupboard) are often left unsealed, whilst visible

penetrations in the same dwelling have been sealed. This suggests

a lack of understanding of the importance of these areas, withthe selection criteria being cosmetic appearance, rather than good

Figure 19Leakage at penetrations through external walls.

Penetrations into the roof space provide further examples of direct

air leakage. Figure 20 demonstrates some common examples, with

air movement around and through electrical penetrations and around

plumbing penetrations in inaccessible areas at the back of a cylinder

cupboard.

Figure 20Leakage at penetrations through the top-floor ceiling.

Observations suggest that one of the biggest contributors to overall air

leakage can be air movement via the soil stacks. The importance of this

leakage path derives from both the number of times it occurs and also

from the apparent speed and volume of airflow relative to other leakage

paths. Where the soil stacks are sited internally, leakage can occur at the

ground floor termination or can provide a direct infiltration route from

the inside of a dwelling to the loft space (Figure 21)



11


components abut masonry elements (see Figure 24). Dissimilar

materials may possess vastly different drying rates and associated

shrinkage, and gaps readily appear, particularly where a less flexible

sealant (such as decorators’ caulk) has been used. It is therefore

important that when used, sealants and other products such as

expanding foam are correctly specified and applied to help ensure agood bond is achieved.

Figure 24Service penetration through an external wall, linking the void behindthe plasterboard to the external cavity.

Possibly the most important point to note about secondary sealing

is that although it can provide some short-term benefit in reducing

air leakage, it may not provide a robust long-term solution. It can

assist in achieving the desired result in a pressurisation test, but

airtightness performance may soon deteriorate upon occupation,

and post-occupancy reparatory works can be costly. In addition, if a high standard of airtightness is necessary to achieve the design

performance of a dwelling, any such deterioration is likely to impact

on the effectiveness of an MVHR system if used.

airtightness, when deciding to seal or not. An example is shown below

in Figure 23. The positioning of the shower tray creates a potential air

leakage path from beneath the shower tray, through the metal stud

partitioning, directly into the ventilated loft-space. In other cases the

air leakage path if often into any number of inter-connected voids

that eventually lead outside. The waste pipe penetration beneaththe shower is unlikely to be sealed creating additional links between

the intermediate floor void, service void and partition wall void,

exacerbating the problem.

Figure 23Voids beneath a shower tray linking adjacent cavities and contributingto complex air leakage paths.

Air leakage through service penetrations can be minimised by ensuring

that all penetrations are appropriately sealed where they pass through

the air barrier. This can be achieved by using gaskets to provide an

airtight seal around pipes.

Secondary sealingSecondary sealing is the process whereby visible gaps in surface

finishes are sealed to limit air movement within construction voids,

such as behind plasterboard dry lining. In most cases, the sealing

provides a secondary defence against air leakage and does not involve

sealing at the primary air barrier.

Anecdotal evidence from a very small sample of reasonably airtight

dwellings at Stamford Brook (see [12]) found that the impact of

secondary sealing on airtightness can result in a reasonably significant

temporary reduction in air leakage, typically between a 10 and 30%

improvement over a parged masonry dwelling with no additional

sealing. However, more importantly, the work also found that secondary

sealing can be prone to degradation over a relatively short time period.

The main reasons for this sudden deterioration being inadequate surface

preparation and usage of inappropriate sealing materials.

After just one heating season drying, shrinkage and settlement gaps

at the intermediate floor perimeter were observed around the sealant

used to seal between the intermediate floors and the skirting board.

Surface preparation in these areas is key, as cracks can become wide

enough to exceed the adhesive and elastic properties of the sealant.

Dust, if not removed from this junction prior to application of the

sealant can result in premature failure of the sealant . Large gaps

are also often observed at failed seals between adjacent materials

with differing physical properties, most commonly where wooden



12

Achieving air permeabilitybelow 5 m3/(h.m2) @ 50Pa

The target air permeability which is set in order to construct dwellingsthat achieve an air permeability below 5 m3/(h.m2) @ 50 Pa will

depend on the consistency with which air permeability can be

achieved in practice. This target is determined by the failure rate

that is deemed acceptable by the individual builder and the resulting

distribution of air leakage. In order to explore the likely target that

will be required to consistently achieve an air permeability below 5

m3/(h.m2) @ 50 Pa, the existing distribution of air leakage measured

by the NHBC [7] has been scaled using a simple model developed by

Lowe, Johnston & Bell [5].

Figure 25 illustrates the resulting air leakage distribution assuming a

failure rate of less than 5% and a maximum air permeability target of

5 m3/(h.m2) @ 50Pa. The resulting average air permeability rate that

would need to be achieved by testing the dwelling would be around

3.6 m3/(h.m2) @ 50Pa. Achieving this average will be demanding as

this level of air permeability is currently very tight by UK standards.

Although air permeability of 3 m3/(h.m2) @ 50Pa or lower have been

achieved in a number of UK dwellings, the numbers involved are small,

with the majority of the dwellings tending to be one-off constructed

by fastidious individuals. The challenge will be to replicate these levels

of airtightness in typical volume housing.

In recent years good progress has been made towards this goal, withthe large housing developers learning invaluable lessons following the

introduction of pressure testing in the Building Regulations.

Figure 25Distribution of air leakage rates assuming a maximum air permeabilitytarget of 5 m3/(h.m2) @ 50Pa and an initial failure rate of less than 5%.

450

400

350

300

250

200

150

100

50

0

0 - 1

1 - 2

2 - 3

3 - 4

4 - 5

5 - 6

6 - 7

7 - 8

8 - 9

9 - 1 0

1 0 - 1 1

1 1 - 1 2

1 2 - 1 3

1 3 - 1 4

1 4 - 1 5

1 5 - 1 6

1 6 - 1 7


N o . o f d w e l l i n

g s

In order to consistently achieve an average air permeability of around3.6 m3/(h.m2) @ 50Pa in typical UK volume housing, a fundamental

rethink of the airtightness design of new UK dwellings will be required

that incorporates the identification of a continuous and robust

primary air barrier at the design stage. This is likely to result in a

move away from current practice in masonry construction where little

attempt is made to explicitly identify the primary air barrier, resulting

in the external wall air barrier defaulting to the plasterboard dry-lining. Alternative solutions for the external and party wall air barrier

are available, such as a wet-plastered internal finish, a mechanically

applied plaster finish or the extension of the acoustic parge coat that

is applied behind the dry-lining on party walls to all external walls.

However, changes to current design and construction practice are

unlikely to consistently achieve the required levels of airtightness

on their own. Instead, it is likely that these changes will have to be

coupled with changes to the way in which the design and construction

is tested, managed and monitored.



13


Stamford Brook Case StudyAn example of the variation in air permeability that can be achieved

in a large masonry housing complex was observed at Stamford Brook

near Altrincham, Cheshire (see [12]. The development comprisesover 700 masonry cavity dwellings designed to an energy efficiency

standard some 25% to 35% in advance of the 2002 Building

Regulations for England and Wales (10% to 15% in advance of the

2006 regulations). As part of the standard, a demanding maximum

air permeability target of 5 m3/(h.m2) @ 50Pa was set for the

dwellings. The main airtightness strategy adopted at Stamford Brook

was the application a thin 2~6mm cementitious or gypsum-based

parging layer (see Figure 26) to the inner leaf blockwork on all of

the external and party walls. This linked to the air barriers formed

by the plasterboard lining to the uppermost ceiling and the in-situ

reinforced concrete suspended ground floor. The purpose of the

parging layer was to decrease the air permeability of the blockwork,

by filling any remaining gaps in perpends and bedding layers, and to

provide conceptual clarity of where and what the primary air barrier

was on the inner leaf blockwork. Other airtightness measures adopted

included the use of timber head plates over the top of the head

channel in top floor partition walls, the installation of window and

doors that incorporated high quality casement and trickle vent seals,

the sealing of electrical ceiling penetrations to timber supports prior

to dry lining, and the installation of plywood heads to service voids

and risers to constrain air movement between the service voids and

the roof space.

Figure 26Parging layer.

As well as the adoption of a number of physical airtightness measures,

the management teams, site operatives and sub-contractors were

also provided with a comprehensive formal training package on

the requirements and procedures for ensuring airtightness. Thetraining covered the airtightness design measures for each of the

different on-site trades to ensure that all operatives were aware

of the construction requirements and the consequences of poor

workmanship. In addition to this training, informal feedback was

provided to the developers after each individual pressure test.

In total, 44 dwellings were pressure tested at Stamford between

February 2005 and June 2007 (see Figure 27). The air permeabilities

of the tested dwellings ranged from 1.8 to 9.7 m3/(h.m2) @ 50Pa.

Although the mean air permeability of the dwellings tested was 4.5

m3/(h.m2) @ 50Pa, which is below the maximum air permeability

target of 5 m3/(h.m2) @ 50Pa set for the dwellings, 14 of the 44

dwellings (32%) achieved air permeability in excess of 5 m3/(h.m2)

@ 50Pa. A closer inspection of the results revealed that the best air

permeability results were achieved in the less complex apartments

and 2-storey dwelling types, whilst the worst results were obtained

in the more complex 2½ storey room-in-roof dwelling types. The

reasons for the difference in performance were felt to be attributable,in the main, to specific design issues that were particular to the 2½

storey room-in-roof dwelling types. These details related to continuity

of the air barrier around the junction between the wall and sloping

section of ceiling

Figure 27Distribution of air leakage rates at Stamford Brook.

N o . o f d w e l l i n g s

14

12

10

8

6

4

2

0

0 - 1

1 - 2

2 - 3

3 - 4

4 - 5

5 - 6

6 - 7

7 - 8

8 - 9

9 - 1 0

1 0 - 1 1

1 1 - 1 2

1 2 - 1 3

1 3 - 1 4

1 4 - 1 5

1 5 - 1 6

1 6 - 1 7


Analysis of the results by test date also reveals some interesting results.

The first pressure tests undertaken on the dwellings between February

and May 2005 resulted in air permeability of between 2 to 3 m3/(h.m2)

@ 50Pa. However, by April 2006 the mean air permeability results had

drifted upwards to over 5 m3/(h.m2) @ 50Pa (see Figure 28). There were

a number of possible reasons for this, such as a shift in focus away from

airtightness, inadequate quality control procedures, training issues and



14

changes in personnel. Measures were subsequently taken to address

these issues resulting in a significant improvement, particularly in the

performance of the more complex 2½ storey dwellings.

Figure 28

Trend in Pressure Test Results: February 2005 to May 2007.

12

J a n - 0 5

A p r - 0 5

J u n - 0 5

S e p - 0 5

N o v - 0 5

F e b - 0 6

A p r - 0 6

J u n - 0 6

S e p - 0 6

N o v - 0 6

F e b - 0 7

A p r - 0 7

J u l - 0 7

10

8

6

4

2

0

Test Date

M e a n A i r P e r m e a b i l i t y ( m 3 / ( h . m

2 ) @ 5

0 P a

)

Apartment

2 1/2 storey semi/end terrace

2 storey detached

2 1/2 storey mid-terrace

2 storey semi/end terrace

3 storey semi/end terrace

2 storey mid-terrace

3 storey mid-terrace

2 1/2 storey detached

In conclusion, the results from Stamford Brook demonstrate that air

permeability’s of less than 5 m3/(h.m2) @ 50Pa can be achieved in

masonry cavity construction, even in dwellings of relatively complex

form, and permeability’s as low as 2 m3/(h.m2) @ 50Pa are possible.

However, consistently achieving such levels of air permeability not

only relies upon the appropriate application of the technology, but

also depends upon well-managed processes and procedures on site.



15


Some common air leakage and ventilation paths

1 Under floor ventilator grilles and floor/wall junction

2 Gaps in and around suspended timber floors

3 Leaky windows or doors

4 Pathway through floor/ceiling voids into cavity walls and then to outside

5 Gaps around windows

6 Gaps at the ceiling-to-wall joints at the eaves

7 Open chimney

8 Gaps around attic hatches

9 Service penetrations in ceiling

10 Vents penetrating the ceiling/roof

11 Bathroom wall vent or extract fan

12 Gaps around the bathroom waste pipes

13 Kitchen wall vent or extractor fan

14 Gaps around kitchen waste pipes

15 Gaps around wall-to-floor joints

16 Gaps in and around electrical fittings in hollow walls.

Table 2Air permeability standards

Maximum air permeability (m3/hm2) at 50 Pa

Approved Document L1A of the Building Regulations (England and Wales), Technical booklet F1 (Northern Ireland)and Building (Scotland) Regulations 2004 technical handbook section 6: energy – poorest acceptable standard

10

Energy Saving Trust (naturally ventilated) 5

Energy Saving Trust (mechanically ventilated) 3

The Netherlands 6

Germany (air changes per hour at 50 Pa) 1.8-3.8 (n50 h-1)

PassivHaus (air leakage rate) ‹1

Super E (Canada) (air changes per hour at 50 Pa) 1.5 (n50 h-1)



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References1 ATTMA (2007) Technical Standard 1. Measuring Air Permeability of Building Envelopes [Internet]. Airtightness Testing and Measurement Association.

Issue 2, 13th July 2007. Available from: http://www.attma.org [Accessed 7th April 2009].

2 BSL (2009) Private communication.

3 CLG (2009a) Proposals for amending Part L and Part F of the Building Regulations – Consultation.

Volume 3: Proposed technical guidance for Part F [Internet] London, Communities and Local Government. Available from|:http://www.communities.gov.uk/publications/planningandbuilding/partlf2010consultation [Accessed 20th July 2009].

4 CLG (2009b) Proposals for amending Part L and Part F of the Building Regulations – Consultation.

Volume 2: Proposed technical guidance for Part L [Internet] London, Communities and Local Government. Available from|:

http://www.communities.gov.uk/publications/planningandbuilding/partlf2010consultation [Accessed 20th July 2009].

5 LOWE, R. JOHNSTON, D. & BELL, M. (2000) A Review of Possible Implications of the Introduction of Mandatory Pressurisation Testing for New

Dwellings in the UK. Building Services Engineering Research and Technology (BSER&T), Volume 21, No. 3, pp.27-34.

6 MILES-SHENTON, D., WINGFIELD, J. & BELL , M. (2007) Evaluating the Impact of an Enhanced Energy Performance Standard on Load-Bearing Masonry

Construction – Interim Report Number 6 – Airtightness Monitoring, Qualitative Design and Construction Assessments , PII Project CI 39/3/663. Leeds, UK,

Leeds Metropolitan University.

7 Defining a Fabric Energy Efficiency Standard for Zero Carbon Homes. Zero Carbon Hub, November 2009.

NHBC (2008) NHBC’s Technical Newsletter - Standards Extra 41. May 2008, Milton Keynes, UK, National House Building Council (NHBC).8 Building Regulations, The Building Regulations 2000 Approved Document Part F: Ventilation. 2010 Edition.

9 Building Regulations, Approved Document L1A: Conservation of Fuel and Power in New Dwellings. 2010 Edition.

10 ODPM (2006c) The Building Act 1984: The Building and Approved Inspectors (Amendment) Regulations 2006 (SI2006/652),

London, UK, The Stationary Office.

11 OLIVIER, D. (1999) Air Leakage Standards. Unpublished DTLR Report.

12 WINGFIELD, J. BELL, M. MILES-SHENTON, D. SOUTH, T. and LOWE, R. J. (2008) Evaluating the Impact of an Enhanced Energy Performance Standard on

Load-Bearing Masonry Domestic Construction. Report Number 8 - Final Report. Lessons from Stamford Brook: Understanding the Gap between Designed and

Real Performance. PII Project CI 39/3/663. Leeds, UK, Leeds Metropolitan University.

13 WINGFIELD, J. MILES-SHENTON, D. and BELL, M. (2009) Evaluation of the Party Wall Thermal Bypass in Masonry Dwellings . Leeds, UK, School of the Built

Environment, Leeds Metropolitan University.

14 DETR (2000) Review of Part L of the Building Regulations for Energy Conservation - Air Leakage Statistics for New Dwellings . London, Department of the

Environment Transport and the Regions Building Regulations Advisory Committee.

Acknowledgements:We gratefully acknowledge the help and advice given by the Home Building Federation (HBF) in the production of this document.

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