Getting your building airtight to improve energy efficiency
With the new targets set out by the Government to reduce carbon emissions and improve the efficiency of the UK’s buildings, all aspects of energy demand within a building, and in particular heat loss, need to be looked at.
In the past homes have been ventilated via natural air ventilation (both controlled, for example via air bricks, and uncontrolled via draughts), but this has been at the cost of high energy consumption for space heating. The introduction of stricter standards and regulations around efficiencies has now led to an increase in the need for higher levels of insulation, increased building air-tightness and mechanical ventilation requirements.
As insulation requirements have increased, so too has the proportion of heat loss resulting from cold draughts. For highly insulated buildings heat loss resulting from gaps and cracks in the construction can be very significant indeed.
Build tight, ventilate right
‘Build tight – ventilate right’ was a concept addressing airtightness. It was first put forward in a paper from 1992, which was shared by the BRE and incorporated into the new Draft Building Regulations in 1993.
The proposition stated that dwellings should be designed and constructed to be as airtight as practically possible, but that they must also incorporate a ‘planned’ ventilation strategy. The paper emphasised that a building cannot be too ‘air-tight’, but it can be under ventilated.
It is common knowledge now within construction that poor ventilation is a serious issue. Excessive condensation can cause mould growth, leading to cosmetic and structural damage to the fabric of a building and can create extremely poor indoor air quality, which can lead to potential health issues for the building’s occupants.
Ventilation requirements
There is now a requirement for newly constructed buildings to pass an air test. This testing, and subsequent targets for air leakage, have been gradually phased in to cover just about all developments, under Parts L1A and L2A of the Building Regulations.
The assessment is done by air permeability testing – or air tightness, air infiltration or blower door testing – which measures the air leakage rate per hour per square metre of building envelope. The lower the air permeability rate, the more challenging it will be to pass.
The design maximum from Part L is 10m3/h.m2, but 5m3/h.m2 is the most common figure used within energy calculations. For buildings with mechanical ventilation, below 3m3/h.m2 is recommended.
The importance of a “continuous air barrier”
To address the problem of a low design air permeability rate within a building, a continuous air barrier must be included within the detailed design. An air barrier is essentially a combination of materials and assemblies that restrict or prevent the passage of air through the building thermal envelope.
Educating and reviewing the work of subcontractors
At every stage, from design and development to the build, all involved need to be informed about the importance of airtightness. It is crucial to make the design team, site management, trades and site labourers aware of their roles in achieving an airtight building. Airtightness should be referenced in subcontracts.
Site operatives must be clear on the location of the air barrier and its importance, and work should be sequenced to allow sealing to be carried out as the building is constructed – this will avoid having to carry out remedial measures late in the project.
Common leakage
Some of the areas that must be considered in design and build are:
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Gaps around pipes, cables and boxing: Make sure all holes around pipes and cables where they penetrate the fabric are sealed – including gaps within intermediate floors, partition walls, boiler and cylinder units and toilet waste pipes as these could be leakage points for external air that has worked its way around the fabric. Sometimes gaps are concealed behind carpenters boxing – these should be permanently filled prior to carpenters boxing being installed.
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Eaves cupboard doors and loft hatches : As these separate the heated dwelling from an unheated space they should be treated as external doors with the same level of sealing and latches.
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Blockwork walls: these are permeable and, if left untreated, will not create an effective air barrier. Using wet plaster or adding a scratch coat prior to plaster-boarding will make this airtight.
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Plasterboard: Drylining is notoriously susceptible to air leakage. Ensure that the plasterboard is continuous (i.e. there are no holes or gaps behind fitted units, sinks/baths, etc). When drylining directly on to an external wall the plasterboard should be mounted on continuous ribbons of plaster or adhesive around all the edges rather than dabs. Ensure the joints between boards are sealed and the plasterboard is correctly detailed at joints, corners, reveals and window sills.
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Skirting: drafts and free air that gather and pass behind plasterboard will enter the dwelling at skirting level. Any areas of missing skirting can cause leakage and any gaps at the base of skirting should be filled with mastic or caulking.
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Fitted units : there are usually gaps and holes to be concealed behind fitted units, such as in kitchens, wardrobes and bathroom units. These should be permanently filled prior to installation.
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Mastic and caulking: this should not be relied upon as a permanent air tight seal. The passage of air flow should have been tackled prior to this last line of defence.
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Shadow gaps: the concealed joint between the ceiling and the external wall must be sealed.
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Light fittings & Plug sockets : holes around light fittings and pull cords in the ceiling need to be sealed. Downlights and plug sockets must be installed prior to the air leakage test being carried out.
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Lift doors: these are usually ventilated to the outside and should be treated as external doors.
