A thermal insulator is a material that opposes the transfer of heat between areas of different temperatures. Every material insulates a building though the variation between materials is vast. At its most basic, if you have two spoons in a pan of boiling water, one wooden and one metal, it is possible to pick up the end of the wooden spoon but not the metal spoon. Heat has been rapidly transported up the handle of the metal spoon while the heat from the boiling water has only slowly traveled up the wooden spoon.

Materials such as glass wool or corkboard are specifically placed within a building to reduce the heat transfer rate. These materials generally play no part in the structural integrity of the building yet provide the majority of the insulation. Other materials within the building structure also have an effect on the insulation properties of a building along with air gaps. The design type and the quality of construction also have an influence on the overall insulation properties of a building. It is therefore necessary to calculate an overall rating for walls, roofs, floors and windows to judge the amount of heat lost from a building.

These calculations are fairly complex and are also case specific. Each building therefore requires to be analysed using the full plan of material and construction technique to ascertain an exact value for the insulation properties of each building. It is however possible to give an estimate of what the insulation value of a building is from a general specification list. The figure that is created is designed only to give a rough idea as to the actual value and may differ dramatically if a non-standard construction technique is used.