Amendments to Part L of Building Regulations 2006 extend the requirement – which originally came into effect from 1st October 2003 - that reasonable provision would be a test result showing that air permeability does not exceed 10m³/hr/m² at an applied pressure difference of 50Pa.

It is often difficult for those not experienced with air leakage specifications to understand fully what an air leakage rate means in terms of envelope construction when it is expressed in terms of Xm³/hr/m² @ 50Pa. To try to overcome this, it is possible to interpolate equivalent envelope leakage areas for a given envelope area and leakage rate. The following table outlines some typical approximate leakage areas for different envelope areas and leakage rates: -

Such figures tend to be more meaningful to contractors, although it must be emphasised that the total equivalent leakage area will generally be made up of a significant length of crackage rather than a few large areas.

Computer modelling can be undertaken to demonstrate the potential energy savings that can be achieved if envelope air leakage is reduced. The following examples are based on single-celled industrial-type units with leakage rates of 20m³/hr per m² @ 50Pa for a leaky unit, and 5.0m³/hr per m² @ 50Pa for a low-leakage unit. The models were based on recorded weather data for Heathrow Airport and an internal building temperature of 16°C.

The energy air leakage requirement for the model buildings was calculated to be: -

Minimising the level of leakage not only significantly reduces the energy consumption and associated CO² emissions, but also provides scope for reducing the capital cost of heating and ventilation plant due to the potential for plant downsizing.