Part L requires that uncontrolled air leakage in all new buildings is reduced to an acceptable level and that the results of air permeability tests (carried out by registered competent companies) are taken into account when calculating the energy performance of a building.


The design air permeability used in the TER (Target Emission Rate) calculations is 10m3/h-m2 @ 50Pa pressure. However, it will probably be necessary to design for better performance than this to achieve a satisfactory BER (Building Emission Rate). Some designers may therefore use higher standards of air tightness to achieve a BER better than the TER.


Roofing contractors should be aware of this, as to achieve higher levels of air tightness during construction will demand a higher standard of workmanship, which will be tested at completion and if the test fails may require expensive remedial work and re-testing for which the roofing contractor may be liable. For example eaves details are a common path for air leakage.