Air tightness testing is a mandatory requirement of Part L of the building regulations for a new build property, whether residential or commercial. All new build buildings must be tested.  

The building control dealing with your project will require a final ‘As built SAP calculation’ or ‘SBEM’ (SAP-Domestic), (SBEM-Commercial), a copy of the air test certificate and EPC to sign off your building. 

The air tightness test of a building is guided by a set target within the energy calculation (SAP or SBEM). The overall calculation determines the target figure. This target figure must be met or bettered to pass the air tightness test. 

Currently, the maximum leakage rate allowed within the energy calculation is 10m3/(hr*m2) @50 Pa. 

10 metres cubed per hour per metres squared at 50 pascals of pressure, which in layman’s terms means the building can leak no more than 10 cubic metres (10m3) of air per hour (hr) when the building pressure is at 50 pascals (@50 Pa). 

The leakage rate is measured against the total metres squared (m2) of the envelope's surface area. The buildings envelope is the total surface area of the floor, external walls and roof. Although the maximum leakage rate allowed within the energy calculation is 10m3/(hr*m2) @50 Pa, this number is usually lower, as it is common for the energy assessor to lower the target score to enable the calculation to pass. 

The better the final air tightness test result, the better the energy calculation, which in turn means the final EPC (Energy Performance Certificate) will have a higher rating. 

Not only will your EPC have a higher rating, but your building will also run more efficiently, your heating and lighting costs should be less, and the energy efficiency will be better. 

The building should be tested as soon as it is complete, as all the final finish has been completed and the best chance of a successful result is possible. Once the property becomes occupied and lived in the fabric of the building will change.  

The more advanced the build and final finish, the better the result will be. Most leakage points will be sealed, the better the finish, the better the result.