ABSTRACT
U-values are unlikely to be entirely accurate, due to: variable atmospheric conditions: solar radiation, humidity and prevailing winds and construction quality including thermal bridging. Manufacturers usually provide tables where specific U-values can be achieved by adopting their materials; these are useful where standard forms of construction are adopted. The thermal insulation of external elements of construction is measured in terms of thermal transmittance rate. Thermal resistances are a combination of the different structural, surface and air space components that make up an element of construction. Thermal bridging is where there is a zone of lower thermal resistance within an external element. The rate of heat loss in this zone will be greater than the rest of the element, i.e. thermal bridge. Airborne resistance varies depending on floor construction, absorbency of materials, extent of pugging and partly on the floating layer.
