Just as a dripping tap can squander thousands of litres of water a year, so too can a poorly designed building squander energy – just a 4mm gap round a badly fitting external door, for example, will waste as much energy as leaving the door open for 20 minutes a day.
It is usually poor design or construction quality that causes loss of energy. Lack of attention to airtightness is one of the most costly factors, which can cause fuel bills to escalate to almost double in extreme cases.
Under the revised Part L Building Regulations, which came into effect on April 6, all commercial buildings and a specified sample of domestic buildings must now be tested for airtightness by a test provider registered by the British Institute of Non-destructive Testing. The onus will be on Building Control inspectors to check compliance.
Airtightness testing was, in fact, included in the 2002 regulations for non-dwellings over 1,000m2, but only as guidance. The most significant change in the new 2006 requirements is that it is no longer guidance but a regulatory requirement.
The responsibility for achieving compliance rests with “the person responsible for carrying out the work”. This would typically be the developer or main contractor, who would probably also arrange and pay for the test.
References to airtightness and how to meet the new requirements are scattered throughout Part L and several additional reference sources are cited (see p72). In Approved Document L1A builders are referred to the TSO publication Limiting thermal bridging and air leakage: Robust construction details for dwellings and similar buildings, while item 50 states: “In accordance with Part L and Regulation 7, the building fabric should be constructed to a reasonable quality of construction so that: the insulation is reasonably continuous over the whole building envelope; and the air permeability is within reasonable limits.”
Bear in mind, however, that insulation is largely hidden, so it is hard for Building Control bodies to check. Part L takes this into account in item 53 of L1A: “… the builder would have to demonstrate that an appropriate system of site inspection is in place to give confidence that the construction procedures achieve the required standards of consistency. For those using the approved details approach… a way of achieving this would be to produce a report demonstrating that the construction checklists… have been completed and show satisfactory results”.
It is clear, in our view, that good design at the outset and a robust quality management system on site will be essential if buildings are to pass the airtightness test; it is not in itself onerous.
The Air Tightness Testing Measurement Association (ATTMA) publication ’Measuring Air Tightness of Building Envelopes’ specifies the test method to be used to calculate the volume flow rate of air supplied per square metre of building envelope area to achieve an internal to external pressure difference of 50 Pascal.
A general minimum level for design air permeability has been set at 10m3/h/m2. There are one or two exceptions where a lower performance is acceptable, but it is more likely that the designer will set a higher performance, in order to reach the target emission rate (TER). Social housing providers and others may require an airtightness rating of 3-5m3/h/m2, so robust quality management on site will be paramount.
Every non-dwelling building is required to be tested, with some exceptions:
l Buildings under 500m2 may wish to accept a poor default value of 15m3/h/m2 but there will be a trade-off elsewhere.
l Factory-made modular buildings may prove their performance using an independent, approved test body to conduct test programmes both in the factory and on site.
l Large and /or complex buildings will sometimes be impossible to test and therefore an independent person will have to be engaged to undertake a detailed programme of design development, component testing and site supervision to provide the necessary confidence.
All extensions to existing non-dwellings are treated as for new build, providing they are:
l greater than 100m2;
l and greater than 25% of the total useful floor area.
For dwellings where ‘Approved Construction’ details have been adopted one test should be carried out on each dwelling type on each development. Separate blocks of flats are treated as separate developments. Each of the following constitutes a change of dwelling type:
l change in method of construction – for example, timber frame and brick and block;
l ground floor, mid floor and top floor flats;
l end of terrace and mid-terrace;
l semi-detached and detached;
l significant changes in building floor area.
Where the ‘Approved Construction’ details have not been adopted, then each dwelling type is to be tested as above but with an increased sampling regime as follows.
For developments with one or two units, one unit must be tested (or both if they are dissimilar in arrangement or construction type). The only exceptions are if the identical construction has been built by the same builder within the last 12 months and successfully passed the airtightness test; or a poor default value of 15m3/h/m2 is accepted in the SAP 2005 calculations.
It is the Building Control body who will select the units to be tested in line with the defined sampling regime. On larger developments, units will be chosen from the first completed batch, to ensure that mistakes can be rectified and lessons applied to later batches.
In our experience remedial work can be costly. It has never been more important for the building industry to get it right first time – and that starts on the drawing board. y
l For more information contact Michael Handley of
Chiltern Dynamics on 01494 569824 or e-mail mhandley@chilternfire.co.uk.
