News Archive
Suck it and see
BY RICHARD PARTINGTON
Part L stipulates that all buildings with a gross floor area greater than 1,000m 2will need to be tested to a recognised procedure devised by the Chartered Institute of Building Engineers (CIBSE) to demonstrate compliance with the standard. The implications for designers, developers and builders are significant.
Airtightness has been promoted as a key issue for conservation of energy by organisations such as the Building Research Establishment (BRE) and the Building Services Research and Information Service (BSRIA); more than 1,000 buildings have been tested in the past 15 years. The principles are generally well understood, but there have been some alarming measured results, with buildings exhibiting high leakage rates.
Studies of UK office buildings, conducted by the BSRIA, show that the construction industry's record is poor and standards are not improving. In a 12-building sample, the post-1990 buildings (average leakage 24m 3/hr/m 2) performed worse than the pre-1990 buildings (average leakage 18 m 3/hr/m 2). The target in the building regulations will be 10 m 3/hr/m 2 at 50Pa pressure.
Admittedly, the survey sample was small and not too many generalised results should be drawn, but for some of the larger buildings in the sample, openings in the structure were cumulatively equivalent to an area of 5m 2, about the same as leaving a pair of double doors open permanently.
The government is clearly expecting the building industry to take time to adjust to the new requirements and has provided an interim period during which 'reasonable' compliance must be reached. The building industry will need that time because, though it may be true that random holes are not generally designed into buildings, poor design can contribute to failures as much as poor workmanship.
The techniques and materials currently used for sealing structural penetrations around pipework, services and the like may not be suitable for achieving airtightness. For example, unfaced mineral wool insulation is air permeable even when compressed (air leakage rate about 40m 3/hr/m 2) and even the effectiveness of expanding foam has been questioned (see BSRIA TN 1999, Potter).
Similarly, the airtightness of blockwork is highly variable depending on its type and density and specifiers will need to request this data from manufacturers.
Sealed and signed off The testing itself is relatively straightforward. It takes a day or two to prepare the building, taping and sealing openings and ventilation equipment, and half a day or so for testing, given a suitable wind speed.
The main methods are, first, to use dedicated mobile fans plugged into a prepared opening and, second, to use paired axial fans in a door blanking unit for smaller buildings. The full procedure is described in CIBSE TM 23 (2000) and numerous design notes and guides are available on the internet.
If there is a failure on completion, however, the remedial treatment may not be straightforward and is often labour-intensive. The question of whether failure is due to its design or workmanship will inevitably lead to disputes.
It is recommended that the building be tested with positive and negative pressure to 50Pa. In other words, air should be blown into it as well as sucked out.Window and door seals will be more leaky at negative external pressure; their seals will be more tightly closed during internal de-pressurisation.
Sealing tiles Clearly it is beneficial to agree beforehand that the design strategy for air-tightness be robust and buildable.
A key principle for design integrity and buildability is to be clear about where the air seal line will be.
The seal line needs to be visible for site inspection and for access, should remedial treatment need to be carried out.The seal line may follow the inside face of the external envelope but it could also be separate, taking a simpler course on the underside of complicated interfaces for roof profiles.
The section through a typical out-of-town office construction demonstrates the principle. At the time of the test, the seal position must be accessible and visible, not hidden behind ductwork and other constructions or secondary finishes such as suspended ceilings and soffits.
The timing of the test also becomes critical when multi-layered construction is used. If the line of seal is above a suspended ceiling then clearly the test must be carried out before the ceiling layer is installed. A certain amount of co-ordination on site and down-time for subcontractors will occur as the building has to be emptied before testing.
If there is a failure, the leakage paths can be established by using smoke visualisation, either using smoke pencils to locate leakage paths or by using smoke generators and observing the egress points from the outside of the building.
If implementing the new standards causes some consternation and readjustment for UK construction it is worth reflecting on our position relative to other European countries.
In Sweden, for instance, mandatory testing is no longer required because industry-wide construction standards and workmanship ensure the highest levels of airtightness. The BSRIA comparison quoted above concludes: 'The average leakage of UK factory/warehouse buildings is 35.86 m 3/hr/m 2and the average air leakage of Swedish factory/warehouse buildings is 4.37 m 3/hr/m 2', more than twice the objectives of the new regulations. There is clearly some catching up to do.
Richard Partington is the principal at Richard Partington Architects. He can be contacted on 020 7619 7400 or visit www.RPArchitects. co. uk
© Architect's Journal 28/02/2002
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