Thermal insulation is specified to control three components of heat transmittance through the building fabric:

• Conduction of heat through building fabric
• Convection via air movement
• Radiant transmission, typically through glass but also through other elements of building fabric.

High levels of insulation and air-tightness are an absolutely critical element of the low-energy, “Passivhaus” concept, but elsewhere, enhanced insulation is not necessarily seen as being a key element of a sustainability strategy.