Thermal bridging

A measure of thermal transmittance, the rate of heat flow through unit area of a building element per unit temperature difference between inside and outside, expressed in watts per square metre kelvin. Lower U-values indicate better insulation, and limiting values for roofs and other elements are set by national building regulations.

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.

The rate at which a material conducts heat. Polymers and cellular foams have low conductivity and act as insulators, while metals conduct readily, a contrast that matters in roof build-ups, battery enclosures and mould tooling.

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.