When earlier this year Chiltern Dynamics offered free seminars to explain air tightness to Building Control at their own premises, places were snapped up. One of several frequently asked questions was: ’What is the pass rate?’
Head of Energy Tom Gregory said, ‘While there is indeed a notional pass rate, we prefer to emphasise that meeting the minimum requirement is often not good enough.’
According to Approved Document L, a ‘reasonable level’ of air permeability for tested buildings is 10m3/(h.m2) at a pressure difference of 50Pa, but if buildings are to achieve their Target Emission Rate (TER), based on SAP or SBEM calculations, a score of 10 is often ‘simply not good enough’, Mr Gregory warned. ‘ A 10 is quite leaky, the equivalent of a 20p piece-sized hole for every square metre of the building envelope.’
How buildings were selected for testing was another area that generated discussion. ‘The advice is that buildings should be randomly selected, but it is more important in our view to avoid persistent errors by testing the first completions.’
Chiltern Dynamics also advises Building Control officers to review the test certificate to ensure that any sealing carried out ahead of the test was in accordance with guidelines. ‘Sometimes whole sections of a building can be sealed up to enhance performance, but this skews the test result and should not be allowed to slip through.’
PDF: Building Control – key questions on air tightness answered
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