Thermal efficiency, walls and insulation

Buildings often feel too warm in the summer and too cold in the winter. In the summer, energy transfers to the building from its warmer surroundings. In winter, energy transfers from the building to its colder surroundings. It costs money to keep a building warm, so various steps are taken to reduce the rate at which energy is transferred from it by heating.

Thermal conductivity

The thermal conductivity of a material is a measure of how quickly energy transfers through it by heating:

  • a material with a high thermal conductivity transfers energy quickly
  • a material with a low thermal conductivity transfers energy slowly, and may be used as a thermal insulator

A building with walls of a high thermal conductivity will cool down faster than one with walls of a low thermal conductivity. The graph shows an example of this difference.

A graph shows the temperature inside a building against time. The graph shows that lower thermal conductivity retains heat for the longest time.Cooling in buildings with walls of different thermal conductivity

Reducing unwanted energy transfer

There are two main ways to reduce unwanted energy transfers by heating:

  • use insulators ‐ materials with low thermal conductivity
  • use thicker materials

The table shows some ways in which this is achieved in homes and other buildings.

Cavity wallA gap between two brick walls contains air, which has a lower thermal conductivity than brick, however the air is free to move so some energy is transferred into the roof space.
Cavity wall insulation The gap between two brick walls is filled with material that has a lower thermal conductivity than air. This is because it contains trapped air which cannot transfer energy by moving.
Loft insulationA thick layer of material with a low thermal conductivity reduces the rate of heat transfer from the ceiling into the roof space.
Double glazed windowsA gap between two panes of glass contains air or another gas, which has a lower thermal conductivity than glass.