Thermal conductivity

The fractional change in a material's dimensions per degree of temperature change. Mismatched expansion between joined materials, for example a composite bonded to aluminium, generates thermal stresses and distortion that must be allowed for in design.

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.