Why cavity walls are a weak point

If your home was built between the 1920s and 1990s, it probably has cavity walls. These are two layers of brick with a gap between them. That gap was originally designed to stop rainwater from reaching the inner wall. But it also allows heat to escape. In an uninsulated home, around a third of all heat loss goes through the walls. Filling that cavity is one of the most effective ways to cut your heating bills. It is quick, relatively cheap, and causes very little disruption. But it is not right for every property. Suitability depends on your wall type, how exposed your home is, and whether you already have damp problems.

Is your home suitable?

First, check the basics. Cavity wall insulation works best on homes with two layers of brick or block and a clear gap of at least 50mm. If your home was built before 1920, it likely has solid walls – no cavity to fill. If it was built after the 1990s, it may already have insulation. You can usually tell by looking at the brick pattern: cavity walls have evenly sized bricks, while solid walls often have alternating long and short bricks. A few other things to consider:

• Wall condition: The outer leaf should be sound, with no missing mortar or cracked render.
• Cavity width: At least 50mm is needed for most materials. Narrower cavities may need foam.
• Existing insulation: If the cavity is already filled, you do not need to do it again.
• Ventilation: Air bricks and vents must remain clear to prevent damp.

If you have a timber-framed home, or one with a steel frame, the approach is different. Always get a professional survey before committing.

The damp and exposure question

Damp is the biggest concern with cavity wall insulation. If your home is in an exposed location – on a hill, near the coast, or facing prevailing south-westerly winds – driving rain can soak the outer wall. Insulation can then bridge the gap and allow moisture to travel to the inner wall. The same is true if you already have damp patches, mould, or a failed damp-proof course. Warning signs to watch for:

• Existing damp or mould on internal walls or skirting boards.
• Crumbling mortar, cracked render, or defective pointing on outside walls.
• Blocked gutters or downpipes that soak the wall.
• Air bricks or vents that are blocked or painted over.

In some cases, a water-repellent insulation material can be used in exposed areas. But it is not a magic fix. If your walls are already wet, adding insulation will make things worse. A thorough survey, including a damp check and a look at local weather patterns, is essential.

What gets injected and how it works

Most cavity wall insulation is blown into the gap through small holes drilled in the outer wall. There are three main types:

• Mineral wool: A fluffy, fibrous material. Good for irregular cavities and generally the cheapest option. It can settle over time, leaving gaps at the top.
• Polystyrene beads: Small beads mixed with an adhesive. They flow well and resist moisture. Good for most homes, including those in mildly exposed areas.
• Polyurethane foam: Expands to fill the cavity. Useful for narrow or uneven gaps. Must be installed carefully to avoid damaging walls.

Installation usually takes two to four hours for a typical semi-detached house. The installer drills a series of holes – usually 22mm in diameter – at regular intervals. The material is blown in, and the holes are filled and matched to the existing brickwork. You may need to move furniture away from walls, but there is no internal mess. After the work, check that all vents and air bricks are still clear.

Costs, savings, and payback

For a semi-detached house, cavity wall insulation typically costs between £500 and £1,500. A detached house might be £1,000 to £2,500. The exact price depends on the size of your walls, the material chosen, and where you live. In terms of savings, government figures suggest you could cut your heating bills by around £150 to £300 per year, depending on your home’s size and your heating fuel. That means payback in roughly three to six years. With energy prices still high, it is a solid investment. Grants may be available through government-backed schemes such as the Energy Company Obligation (ECO) or the Great British Insulation Scheme. These are aimed at low-income households or those on certain benefits, but some installers offer discounted rates. Always ask about eligibility before you sign anything.

Getting it done right

Use a reputable, experienced installer. Do not accept a quote without a survey. The surveyor should check your wall type, cavity width, exposure, and any signs of damp. They should also confirm that your vents and air bricks are clear. Ask for a 25-year guarantee that covers the materials and the installation. After the work, keep an eye on your walls for the first few months. A little damp inside is normal as the building dries out, but persistent damp or mould means you should call the installer back. Finally, remember that cavity wall insulation works best alongside other measures: loft insulation, draught-proofing, and smart heating controls. Together, they can make your home warmer, greener, and cheaper to run.