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Taking control of air leakage


According to the Energy Saving Trust’s Chief Executive Philip Sellwood, almost a third of new homes are still failing to meet energy efficiency guidelines. He told the BBC “… the Government's 'Code for Sustainable Homes' is not being adequately enforced, giving cause for real concern. Our building regulations in the UK are among the toughest in Europe, but they are extremely poorly enforced as far as energy efficiency goes”.

David Arendell, MD of roofing specialist Klober feels the situation in respect of building airtightness gives grounds for even greater concern. He commented, “Despite the fact that the phrase 'Build tight, ventilate right' has become synonymous with the strategy to construct low energy buildings, awareness of how best to achieve airtight construction remains poor. Failing to achieve the right balance between airtightness and controlled ventilation increases the risk of condensation within the roof space. With every upgrade in insulation standards, the risk increases”.

Delays in consultation on Approved Documents L and G have prompted deferment in CSH 2010 until the end of the year. Some house builders have lobbied for a clearer definition of how 'zero carbon' can be achieved, prompting establishment of the Zero Carbon Task Group. There is some evidence to support calls for redefinition as research carried out by the Richard Hodkinson Consultancy, for example, showed that 'PassivHaus' (the Europe-wide Standard with stringent airtightness requirements managed by the BRE and the Energy Saving Trust) would not actually meet even CSH 3. CSH assessment uses the SAP test to calculate energy performance, and for some time there the efficacy of the test, particularly in relation to more thermally efficient buildings, has been questioned.

The requirement already exists for new public sector housing to meet CSH 3 and the impetus towards 'zero carbon' will be given fresh impetus when the equivalent of CSH 3 is incorporated into Building Regulations for England and Wales (similar improvements are planned in Scotland).

Of the nine categories within the CSH method of assessment, 'energy and CO2 emissions' is by far the most significant. This is true in terms of allocation of credits, for which 29 are available, and the final points allocation, 36.4% of total available performance once weighting factors have been taken into account.

The right balance between airtightness and ventilation can be struck without significant addition to building costs. Material choice, however, can greatly influence long-term airtightness. Sheet membrane air barriers coupled with sealants, for example, are more effective than sealants alone, counteracting the effects of buildings (particularly timber frame) drying out. Accredited Construction Details (ACDs), Enhanced Construction details (ECDs) and, in Scotland, the Scottish Ecological Design Association Guide are now available for both warm and cold roof construction. Accredited detail Sheet MCI RE 02, for example, shows a warm roof detail at the eaves using Klober Permo forte vapour permeable underlay and either appropriate tapes or a pre-taped product.

Despite the absence of any CSH equivalent for non-residential construction, airtightness can be important in relation to use of roofing materials such as zinc (if the metal’s underside is unventilated). Abergwynfi primary school near Neath was designed to achieve a BREEAM 'Excellent' rating, with zinc used on a series of circular classroom roofs. Klober Wallint air barrier was installed with compatible sealing tape to meet the specified airtightness.

With current Building Regulation requirements stipulating airtightness of only 7m3/hr per m2 compared with CSH 3 at 3m3, techniques used to achieve it must undoubtedly change. 'The Code for Sustainable Homes and airtightness in roofs' CPD presentation from Klober examines how to 'build tight and ventilate right' within the realms of practical pitched roofing construction. It is a welcome source of information on a subject for which little is otherwise available. For more information go to www.klober.co.uk/airtightness.

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