BRE IP5/06 PDF

Introduction

This Information Paper describes the general principles concerning the modelling of condensation in cold pitched roofs. It describes the system and the processes that have to be modelled, the sources of the climate data necessary to drive a model, and the output parameters that can be used to assess whether a roof is likely to suffer from problems.

Most houses (70–80%) – and some larger domestic-type buildings – in the UK have ‘cold' pitched roofs with insulation on a horizontal ceiling and an accessible, cold loft space above. Heat and moisture are generated by the normal activities of the household within the occupied space of the house below the ceiling, raising the temperature and vapour pressure above those outside. Some of this heat and moisture leaves the house via the loft, passing through the ceiling by a combination of conduction, diffusion and air leakage. Studies have shown that about 20% of the air that enters a house leaves via the roof, and that typically 80% of the water vapour is transported into the roof by air leakage. BRE IP 4/06 discusses the airflow routes and methods for sealing the ceiling.

The procedure specified in BS 5250 for assessing the risks of condensation within structures uses a calculation procedure taken from BS EN ISO 13788. Structures with airflows through cracks and cavities are specifically excluded from the latter standard. As airflow dominates the movement of moisture into cold pitched roofs, assessments of condensation risk using this procedure are not valid. A new method taking account of airflows, is needed.