Following the launch of its report, A practical guide to building airtight dwellings, Neil Smith, Research Manager at the NHBC Foundation explains all you need to know about airtight dwellings.
Airtightness is about creating and maintaining a balance of insulation and ventilation to ensure that the building is energy efficient, and that residents are comfortable. As poor airtightness can account for up to 40 percent of heat loss from buildings, getting this balance right is crucial.
Airtightness has a key role to play in meeting requirements to make the UK’s housing stock more environmentally efficient – the Government has set targets for a 25 percent improvement on current standards for new homes or buildings from 2010, with a further change in 2013 to reduce carbon emissions by 44 per cent on current standards. The aim is to make all new homes zero carbon from 2016, and since 2006 it has been mandatory for a building to pass an air leakage test in order to comply with Part L of the Building Regulations in England and Wales.
PART L
Airtightness is measured as air permeability; that is, the quantity of air (in m3) that leaks into or out of the dwelling per hour, and the lower the value, the more airtight the building. To comply with Building Regulations a minimum performance of 10 m3/(h.m2) is necessary. That said, the air permeability value used for the SAP may be lower than 10 for a particular dwelling, and that lower value would become the air leakage target.
Consultation opened earlier this year on possible changes to Part L. The consultation, which closed on 17 September 2009, proposed increasing the number of properties tested. Further, it also suggested that for those homes not actually tested, a lower level of airtightness should be assumed. In practice this would mean that the untested property is assumed to be 2m3/(h.m2) less airtight than the average result achieved on site. It also encourages sealing the party wall around its edges or the use of full-fill insulation in the party wall cavity to limit the movement of air and associated heat loss.
Phil Rogers, Testing Services Manager at NHBC, said: “In the thousands of tests carried out by NHBC, fewer than three per cent failed to achieve a score of 10. However, of those that do fail, there are some problem areas that crop up more than others. These include service penetrations, especially in bathrooms and kitchens, suspended floors, especially around perimeters, and gaps in the air barrier in the roof area, especially around built-in storage around rooms in the roof. Yet all of these can be easily mitigated if the right measures - such as those outlined in the NHBC Foundation report and NHBC’s air leakage fact sheet - are implemented at the outset.”
The same is true for the millions of existing homes, for which airtightness was not a consideration at the time of their construction. In addition to their draughty floorboards and windows, chimneys are often also a major source of air movement and heat loss.
WHY DOES AIR LEAK?
Air leakage occurs when air seeps through cracks and gaps in the dwelling’s fabric, in a process known as infiltration or exfiltration. The three main factors influencing in/exfiltration are:
Wind: Wind against the dwelling causes pressure differences between the inside and outside. Air is drawn into the dwelling through gaps on the windward face (infiltration) and leaves the dwelling on the leeward face (exfiltration).
Air buoyancy: Warm indoor air is more buoyant than colder outside air (buoyant air rises by convection). This rising effect draws in cooler air from outside (infiltration) which is felt as cold draughts inside. The rising effect increases the pressure inside the dwelling which pushes warm air out of cracks and gaps in the envelope (exfiltration). It is worsened when it is very cold outside and warm inside.
Condensation: Warm moist indoor air that is drawn out through gaps in the envelope may lead to interstitial condensation occurring on cold surfaces in the fabric, which can give rise to mould and even decay.
WHERE DOES AIR LEAK?
There can be many different air leakage paths in a building, which good construction, careful design or refurbishment planned in detail can help to mitigate. Common paths include:
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Gaps between floorboards
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Gaps between doors, windows and frames
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Inadequate seals in loft hatches
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Gaps around soil pipes and flue stacks
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Gaps around recessed ceiling lights or ceiling roses
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Gaps in mortar joints
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Unused chimneys and flues that have not been sealed off.
As airtightness increases, the energy needed for heating (and cooling) reduces, but the need for controlled ventilation to ensure occupant comfort and safety increases.
Therefore, when making a dwelling airtight to prevent air leakage, it is important to also provide adequate ventilation: build tight – ventilate right.
TOPS TIPS FOR ACHIEVING AIRTIGHTNESS
Design stage
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Identify the line of the air barrier on drawings and assign one person to ensure continuity of it throughout the build
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Avoid complex detailing that is difficult to build as it may result in ad hoc air leakage
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Make sure you specify air tight barrier materials
Construction stage
