BRE IP20/01 PDF

Introduction

New materials and methods of construction used in pitched roofing have focused attention on the risk of condensation under roof slates. Little is known about the temperature and humidity variations in the batten void (the space directly beneath the slates). Using insulation at rafter level and breathable membranes may alter the conditions that have existed with previous forms of roof construction. Breathable membranes allow moisture to pass out of the roof void (and into it) but resist the passage of liquid water. Water vapour that passes through the membrane is assumed to naturally disperse through gaps between the slates. If it does not disperse, there is a risk of condensation when the external temperature is low.

The effect of condensation on fibre cement slates is dealt with in another Information Paper, IP 19/01[1].

The construction and instrumentation of an experimental roof, which included insulation between the rafters and breathable membranes, are described in the next section. The results of measurements made over 12 months, of the internal and external conditions that generated condensation within the roof, and the moisture content of roof battens, are summarised. This has provided some basic data for comparison with other more traditional types of construction, and enabled assessment of the likelihood of condensation occurring and its significance.