BRE IP4/06 PDF

Typically about 20% of the air entering a house leaves via its loft, adding to the risk of condensation in the loft and reducing the energy efficiency of the roof. This airflow, together with the ventilation of the loft space, is not accounted for in standard diffusion-based methods of condensation risk analysis, which therefore do not apply to cold pitched roofs.

This paper describes the airflow routes between a house and its loft and the steps that can be taken to reduce the flow. The methods that can be used to measure the airtightness of ceilings are discussed and a method is proposed for assessing the condensation risk, taking into account the airflows.

The paper explains why reducing airflow through the eaves does not improve the energy efficiency of a house and gives recommendations for improving the airtightness of ceilings in new houses and in existing homes. Reducing airflow through the ceiling both saves energy and reduces the risk of condensation in the loft