Airtightness testing will add another step to the commissioning process. The targets are not too onerous, but airtight buildings will help meet carbon emission targets, says Nigel Potter, technical development manager at BSRIA
Under the new part l of the new building regulations, there is a strong possibility that all new buildings must be tested for airtightness irrespective of size or sector. The test is designed to reduce the amount of heat lost through air leakage, by forcing contractors to improve the quality of the building envelope. If a building fails the test the builder will have to return and rectify the problem (see “How to get it right first time”, below).
The maximum permitted air permeability would be 10 m3/h/m2 at a pressure of 50 pascals. The legal requirement would be defined in a new regulation and guidelines would be provided for the residential and commercial sectors in the approved Part L documents.
Not all dwellings on a development would need to be tested under the proposal. However, a selection of different types would need to show compliance. Some dwellings may not need a pressure test if the air permeability is assumed to be 15 m3/h/m2, but it would then be difficult to meet the carbon emission target for new dwellings.
For buildings of less than 1000 m2 there would be a period of 18 months when an airtightness target of 11.5 m3/h/m2 would generally be acceptable. However, whatever the airtightness, the result must be fed back in to the National Calculation Methodology (see page 15 or visit www.ncm.bre.co.uk for more information), to see whether the building would meet its carbon emission target. When the building is complete it will receive an asset rating, which determines its energy performance.
If the asset rating of the building as actually built fails the required asset rating, increasing the airtightness would be one of the few options available to bring the building up to the required standard.
Airtightness testing should be undertaken by competent organisations such as members of Air Tightness Testing and Measurement Association and must always be carried out in compliance with ATTMA Technical Specification 1.
It is unlikely that pressure testing will be required for extensions to existing buildings, unless the increase in floor area is over 25% and more than 100 m2. If it is, requirements in the approved document for non-dwellings will apply. This presents a conundrum if the extension can’t be tested in isolation from the existing building: an example is increasing the sales floor area of a superstore. One option is to carry out a pressure test before the work starts and again after completion. The air leakage rate difference gives an air permeability for the extension. The other option would be to upgrade the existing part of the building to a level of 10 m3/h/m2.
The time for an airtightness test varies with building complexity but dwellings and large single cell buildings should take no more than an hour after setting up the equipment. Smoke tests to determine air leakage paths can generally be carried out in less than half a day.
Pressure tests are best carried out several weeks before handover and before suspended ceilings and so on are installed, since it is the internal sections of external walls that are the air-sealed surfaces. This leaves time for corrective action to be taken while the building elements are still accessible.
Leaky buildings represent a very significant waste of energy and a high proportion of the overall heat loss of any building. The target of 10 m3/h/m2 is not arduous. Indeed, there will be many buildings that will adopt lower airtightness targets to meet the overall carbon reduction target required in the regulations. The next round of revisions to the Building Regulations of 2010 will almost certainly have lower airtightness targets in any case, and that is only four years away.
How to get it right first time
The critical aspects to achieving airtightness are good detailing (particularly where different trades meet), careful product selection at the design stage, coupled with good quality assurance during construction.
In the domestic sector, poured concrete, plastered walls, plasterboard, and well-jointed autoclaved aerated concrete blocks do not leak significantly. On the other hand, common concrete blocks, gaps around and through plasterboard, uncollared pipe penetrations, underneath skirting boards, suspended wooden floors and light fittings penetrating the air barrier do leak.
Careful selection of modern double-glazed weather-stripped windows should pose little problem but gaps under and around window boards should be sealed. External doors, patio doors, letter boxes and loft hatches should be specified with adequate seals. Services penetrations, flues and the areas around ventilation openings and so on need to be detailed, preferably with collars, including penetrations to cold roofs and risers.
In the commercial sector, the joints between roof liner sheets need to be sealed to themselves and all wall junctions. Specification of low-leakage smoke exhaust fans and louvres are required. The use of vapour barrier membranes should be avoided, along with perforated acoustic roof liner sheets. Good detailing where different trades meet is critical.
The Building Services Research and Information Association has several publications on the website that address
National Calculation Methodology www.ncm.bre.co.uk