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The implications of poor building details extend beyond increased heat loss and associated compliance with Part L CO2 and fabric energy efficiency (FEE) standards.

Where the internal surface temperature around a building detail (such as a lintel or sill) is excessively low, the risk of surface condensation is increased. This can damage surfaces and lead to mould growth which is linked to allergies, asthma and respiratory infections. This clearly exposes developers to risk and reputational damage.

It is generally accepted that the use of Accredited Construction Details (or similar, such as Constructive Details) sufficiently address the risk of surface condensation. However, where developers choose to use details that are not covered by these standards, without numerically modelling the detail it is not possible to rule out the risk of surface condensation.

We recommend that where a default Ψ-value (the measure of heat flow through a junction detail) is adopted, numerical modelling is still carried out to determine the temperature factor of that detail.[1]

It is our experience that addressing this at the earliest stage of design is crucial, as critical details such as window heads, balconies and ground floor details often fail this assessment unless continuity of insulation has been prioritised in the design. Developers should be particularly aware of the following areas:

  • Balconies which penetrate the primary insulation layer (e.g. cantilever balconies)
  • Concrete frame buildings
  • Refurbishments
  • Any other building detail or construction to which Accredited Construction Details do not apply.


[1] Building Research Establishment Information Paper 1/06 states that this is required.