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. 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. 

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. 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 overall calculation to pass. 

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