Out of the ambition to produce a more energy efficient housing stock, the Scottish Building Regulations have tightened the requirements for the air tightness of new buildings. Now they also need to be tested for compliance with respective certification.
The measurement standard is called ‘q50’, which measures the escaped air [in cubic meters] per hour in relation to the surface envelope area of the building [in square meters], when the house is set under pressure of 50 Pascal. All controlled ventilation openings are excluded from the test. The smaller the value, the more sealed the building.Air tightness of buildings
If the test result exceeds 7 m3/(h m2) at 50Pa it has failed and remedial measures need to be taken.
However if the building is tighter than 3.5 m3/(h m2) at 50Pa, mechanical ventilation (MV) – and in particular MVHR – is a requirement.
Architects have to set a target value for the air tightness test in SAP with a dedicated ventilation strategy. The practical test with certification for Building Control needs to be done when the building is finished, at that point it is difficult to counteract. Problems arise e.g. if the house turns out to be too air tight and no MV has been installed.
Passivhaus: For your interest, the standard for Passivhaus is about 1/10th of the leakage rate than BC requires (n50 <=0.6/h at 50Pa). The test result is expressed in a different way (n50): it is related to the internal volume, not the surface area. n50 and q50 are not directly related. For more information on this subject see: http://www.paulheatrecovery.co.uk/what-is-a-passivhaus/