In most houses built after the 1920s, the external walls are made of two layers with a small air gap or 'cavity' between them. If your home has unfilled cavity walls, a considerable slice of your energy bills will be spent heating the air outside.
Filling the gap between the two walls of a house with an insulating material massively decreases the amount of heat which escapes through the walls. It will help create a more even temperature in your home, help prevent condensation on the walls and ceilings and can also reduce the amount of heat building up inside your home during summer hot spells.
Around a third of all the heat lost in an un-insulated home is lost through the walls. If you have cavity walls, insulating them will make a great difference, and it could save you £130 - £160 a year on your fuel bills.
For more on how cavity wall insulation works, costs, savings and other considerations see the Energy Saving Trust information page.