2.2 Cutting building fabric heat losses

Heat loss mechanisms

Heat energy will flow through any object when the temperature on the two sides is different.

The rate of this energy flow (i.e. the number of watts) depends on:

• the temperature difference between the two sides
• the total area available for the flow
• the insulating qualities of the material.

It is obvious that more heat is lost through a large area of wall or window than a small one, and on a cold day than a warm one. In order to understand how this heat loss occurs, and how it can be minimised, it is necessary to look at the three mechanisms involved in the transmission of heat: conduction, convection and radiation. While conduction is the main mechanism in insulated walls and roofs, all three play an important role in windows.

This section discusses ways of reducing heat loss through windows and walls, and introduces some of the parameters used to describe how well (or badly) objects conduct heat.