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Insulation versus air tightness

by John Boanson
Architect/ NBS Technical Author

Initially in order to make buildings more comfortable, and now to reduce the carbon emissions of our buildings, we have been, for many years, increasing the thickness of wall and roof insulation and adding underslab insulation.

This seemed to make sense, however I recently attended a Part L seminar where one of the speakers compared two recently built superstores. They were built one year apart and the insulation levels on the later store far exceeded those built into the first store. Although the stores were similar in size, had equivalent levels of glazing and rooflights, the more recently built store, despite having better U-values, cost 43% more to heat than the older one.

The maximum acceptable air leakage rate stated in the 2006 editions of Approved Documents L remains at 10m³/hr/m²@50Pa, the exemption from testing for buildings smaller than 1000 m² floor area has disappeared, however there is an option for buildings less than 500m² floor area where an alternative to testing will be to assume an airtightness level of 15m³/hr/m²@50Pa. This value is to be used in the calculation to predict the carbon emissions from the building.

When the level of airtightness is so important to fuel economy why aren't the standards being increased? Perhaps the reason is that the current testing regime has not been enforced as strictly as it should have been. It is argued that this occurred because the Approved Documents are guidance only, however a circular letter dated 4 April 2003 from the Office of the Deputy Prime Minister stated that their view was that there was no satisfactory substitutes to pressure testing. Although the number of buildings being tested has recently increased, pressure testing organisations believe that only around 25-30% of buildings are being tested.

Of the buildings that have been tested a large variation in results has been observed. Industrial/ commercial 'shed' units have tended to have about 95% compliance rate when first tested, with average results falling into the 4-8m³/hr/m²@50Pa. Office/ retail/ school buildings tended to have about 50-65% compliance rate when first tested, with average results 8-12 m³/hr/m²@50Pa.

We obviously still have a long way to go to meet the existing standards, so there may be little point in increasing the targets at the present time.

Where do air leakages tend to occur? The majority occur at the interface points between one trade and another, e.g. wall/ roof junctions, cladding panels attached to out of line steelwork etc. These are the areas that must be concentrated on. The building envelope must contain a continuous air barrier in order to minimize air leakage. Although air leakage rates remain unchanged for the 2005 edition of the Building Regulations, the standard will certainly by higher in the future. In the UK we lag behind many other northern European countries in terms of air tightness of buildings.

Written March 2005, updated January 2007

 

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