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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.
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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.
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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 )
7 (naturalventilation)
5 (naturalventilation)
3 (naturalventilation)
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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How to achieve good levels of airtightness in masonry homes
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.
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How to achieve good levels of airtightness in masonry homes
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.
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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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How to achieve good levels of airtightness in masonry homes
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)
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How to achieve good levels of airtightness in masonry homes
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
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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
Air permeability (m3/h.m2) @ 50Pa)
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.
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How to achieve good levels of airtightness in masonry homes
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
Air permeability (m3/h.m2) @ 50Pa)
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
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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.
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How to achieve good levels of airtightness in masonry homes
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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How to achieve good levels of airtightness in masonry homes
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.