One of the main functions of a building is to isolate the accommodation from the external temperature conditions. The most important structural factor in achieving this function is the actual envelope enclosing the accommodation, even if the envelope has no thermal insulation value, as there is always resistance to the transfer of heat from the outside air to the envelope structure and from it to the inside air, or vice versa. This resistance to heat transfer is known as surface resistance and applies, whatever the thermal properties of the envelope; the actual level of resistance varies depending on the type of surface, so that a polished reflective surface will resist heat transfer and will have high surface resistance compared to a rough surface. Further resistance to heat transfer can be achieved by improving the thermal insulation of the structural envelope, so that resistance to structural heat loss will depend on the surface resistance plus the structural resistance, as explained in more detail in section 3.2. Heat loss does not, however, depend only on structural heat loss, but also on ventilation heat loss through both deliberate and accidental ventilation, as explained more fully in section 3.3.