Breather membrane

A membrane or sheet installed on the warm side of insulation to restrict the passage of water vapour from inside the building into the roof or wall construction, reducing the risk of interstitial condensation. Materials range from polyethylene sheet to bitumen membranes and foil composites, and effectiveness depends on sealed laps, edges and penetrations.

Condensation that occurs within the layers of a roof, wall or floor construction rather than on its internal surface, when water vapour moving through the build-up reaches a plane at or below its dew point temperature. It can degrade insulation, corrode fixings and decay timber, and is controlled through vapour control layers, ventilation and correct layer sequencing.

A roof construction in which the insulation is placed at ceiling level below the structural deck, leaving the deck and the void above it at close to external temperature. The void normally requires ventilation to remove moisture vapour, and this arrangement is discouraged for flat roofs in current UK guidance because of condensation risk.