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technical & practice

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Architect: John Miller & Partners

As part of a comprehensive energy conservation strategy, the design team of the University of East Anglia's Elizabeth Fry building wanted the finished building to be thoroughly airtight. Given that cold spots and draughts caused by unsealed construction are causes of discomfort and wasted energy, sealing the building and designing in managed ventilation flows were prime objectives. 'We set out to achieve 5m3 per hour per m2 at 50 pascals', said Richard Brearley, partner of John Miller and Partners. 'To ensure that we considered the efficacy of the building as a whole, the entire envelope was pressure tested, rather than just testing a room or a single compartment.'

Taking advice from Energy Advisory Associates, the architects included testing clauses and sealing details in the tender documents. 'We specified our performance requirements in the tender documents so that any necessary extra work would be allowed for in the contractor's price,' said Brearley. One of the key lessons learned was that airtightness need not be a difficult issue. In the main, the design team and contractors relied on traditional construction techniques and simple details.

Poor workmanship can easily lead to gaps in construction and to tackle airtightness effectively, contractors need to perform to a higher standard of workmanship than they are used to. For example, normally they do not plaster behind skirting boards or above suspended ceilings, but an effective solution to leakage - utilising wet plaster as the airtight seal - can be achieved if they are encouraged so to do. Generally they must ensure that all junctions are filled, and the most demanding of all the junctions are the window/wall interfaces. At Elizabeth Fry, the architect used a proprietary polythene barrier, tacked to the outside of the window and folded so that air could not pass through the junction of the window and wall. The suitability of this straightforward detail was confirmed in pressure tested mock-ups.

Brearley believes that in traditional construction, there is little extra cost needed to achieve airtightness standards - apart from the cost of the pressure test, nominal additional materials and more exacting supervision - because much of the work is already covered by standard specifications. The finish of the plaster used for sealing does not need to be skimmed to such an even finish as more visible areas of construction, but above ceilings, for example, it will allow ceiling heights to be adjusted at a later date without the need for wet trade involvement.

John Miller and Partners now writes airtightness criteria into most specifications for similar buildings. However, Brearley believes that the benefits of airtightness are still not well understood. Although the payback is not immediately obvious, the user does benefit in the long term from reduced energy costs, and in better standards of comfort and control throughout the life of the building. But even so, it may be difficult to convince clients, especially where the end user and commissioning agency are separated.

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