BWF members such as yourselves frequently use our Member and Technical Helplines for assistance on a wide range of Employment, Health and Safety, Pay and Technical issues. We’re putting up a frequently asked question every Tuesday in order to help members and illustrate the type of advice we are able to provide. If you have any questions that you would like to see featured, we would be happy to assist. This question and many others like it can be found in our Question Centre. If you want further advice on any of the issues, don’t hesitate to ring the helpline or browse our extensive publications library, which includes our Fact Cards and guidance on installing timber windows. This week: Why are new dwellings pressure tested? The requirement for pressure testing of new dwellings is given in Approved Document L1A. They are pressure tested to either demonstrate that they at least achieve an air permeability rate no worse than 10m3/(h-m2) at 50 Pa, or to confirm that they achieve a lower rate if one was used in the SAP calculation that determined the dwelling CO2 emission rate (DER). On each development, sample air pressure tests should be carried out for each dwelling type including each type of apartment in a block of flats. Compliance with the Building Regulations would be demonstrated if: a) the measured air permeability is no worse than 10m3/(h-m2) at 50 Pa; and b) the DER calculated using the measured air permeability is no worse than the TER (target CO2 emission rate). An alternative approach is available for small developments where no more than two dwellings are to be built. Reasonable provision would be; a) to demonstrate that during the preceding year, a dwelling of the same type, constructed by the same builder has been successfully pressure tested; or b) avoid the need for any pressure testing by using a value of 15 m3/(h-m2) at 50 Pa for the air permeability when calculating the DER. However, using this value would require compensating changes to be made elsewhere in the design of the dwelling i.e. lower U-values for the windows. The approved procedure for pressure testing is given in the ATTMA publication, ‘Measuring air permeability of building envelopes.’ During the test, trickle ventilators should be temporarily sealed rather than just closed. Typical air permeability figures for windows can be taken from the standards for the classification of weathertightness. For windows classified in accordance with BS 6375-1:1989 the highest performance rating of 600Pa (or curve C) corresponds to an air permeability rate of about 5.5 m3/(h-m2) at 50 Pa. This is equivalent to a European air permeability class 3 for windows classified in accordance with BS 6375-1: 2004 (or 2009) with class 4 windows having an air permeability rate of about 1.5 m3/(h-m2) at 50 Pa.