Air permeability is an important factor in assessing the overall carbon emission of a building via the appropriate calculation methodology:

• Standard Assessment Procedure (SAP) for dwellings under 450m² floor area – accredited software has been developed to make calculation easier.
• Simplified Building Energy Model (SBEM) for buildings other than dwellings – software still under development

Before construction:
SAP or SBEM is used to calculate a Target Emission Rate (TER) in advance of starting building work. This is based on a range of factors including orientation, glazing, insulation, heating system/fuel and air permeability. The details of the above are entered into the software along with a ‘Design Air Permeability’ what you hope the building will achieve under testing (must be under 10m³/h/m²) or 15 m³/h/m² if an assumed value is to be used.

The TER is effectively the performance the building would have achieved under 2002 regulations, with a 20% improvement factor applied, in line with wider government targets for reducing carbon emissions.

To achieve the TER, you may need to change the specification of products, orientation of buildings, heating system or commit to achieve air permeability lower than the basic regulatory requirement.

If you want to view how you might achieve the TER using your existing methods and products you can do so at www.playtheregs.com

After construction:
The building is then constructed and the specification and configuration of the building may change over the course of the process. The actual details and tested air permeability figure are entered into the software again to give a Dwelling or Building Emission Rate (DER or BER)

The actual DER or BER must be lower or equal to TER.

This is a regulatory requirement under Approved Document L

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