Walls lose more heat than any other part of most houses simply because there is so much of them. Where a home has a hollow cavity between two skins of brick, filling that gap is among the higher-value jobs you can do, and on many houses it is already half the price it looks thanks to grant schemes.
Does your house even have a cavity?
Homes built from roughly the 1920s onwards tend to have two leaves of masonry with a gap between them; older solid-wall houses do not, and need a different approach covered on the solid wall guide. A quick test is the brick pattern: an unbroken run of long bricks usually means a cavity, while a regular pattern of short brick ends shows a solid wall. The wall thickness at a window or door reveal is another clue, as a cavity wall is noticeably thicker than a single-skin one.
What it saves and what it costs
An empty cavity lets warmth pass straight through; injecting mineral wool, beads or foam through small holes in the mortar slows that loss considerably. For a typical semi the annual saving is one of the better ones available from a single measure, and the work takes a few hours with no mess indoors. The holes are made good afterwards and are barely visible once the mortar weathers in.
The damp question
You will hear stories of cavity insulation causing damp, and they are not pure myth: in exposed, wind-driven-rain locations, or where the original installation was poor, moisture has occasionally bridged the gap. The answer is not to avoid it but to use a reputable installer who surveys the property first, checks for exposure and existing damp, and fits a product suited to the situation. A proper survey weeds out the houses that should be left alone.
Check before you pay
Many houses had their cavities filled years ago without the current owner knowing. Before commissioning anything, ask a surveyor to drill a small inspection hole, or check the deeds and any past paperwork. There is no sense paying to insulate a cavity that is already full, and a borescope check costs a fraction of the job itself.