As a rule, almost all domestic buildings erected after about 1920 have cavity walls. One of the easiest ways to check is to look at the brickwork on the outside. In the case of cavity walls, this will usually be in a regular pattern, with just the side of the brick exposed. Older, solid-wall homes will display alternating brick ends exposed on the wall face. Another good trick is to check the width of the brickwork at a window recess. If the wall is more than 260mm deep, it probably includes a cavity. You should be able to spot drilled holes if the wall has already been filled. If all else fails, ask a registered installer to drill in and check for you.

The Energy Saving Trust has some practical information and diagrams (www.est.org.uk) or turn to page 25 of our eco supplement for more on insulation.