Is Cavity Wall Insulation Worth It? Costs, Savings, and Risks

Use our insulation savings calculator for a personalised estimate based on your property type and current insulation levels.

Cavity walls consist of two layers of brick or block with a gap (cavity) between them, which needs to be at least 50mm wide and clear of rubble to be filled. This design, usual in UK homes built from the 1920s onwards, was originally intended to prevent rainwater reaching the inner wall. An unfilled cavity also provides some insulation, but far less than filling it with insulating material.

For an average home with easy to reach walls, the Energy Saving Trust says the job usually takes about two hours. The installer drills small holes through the outer wall at regular intervals, blows insulation material into the cavity through the holes, then fills them. The Energy Saving Trust says the material is usually polystyrene beads.

The process is minimally disruptive. No internal work is needed and no rooms are affected. If the filler does not match the brickwork you may still see where the holes were, but they can be painted over. The main inconvenience is the noise of drilling, which lasts a few hours.

Before paying for a survey, you can often identify cavity walls yourself.

Check the brickwork pattern. If all external bricks show their long face (stretcher bond), the walls are almost certainly cavity. If you see alternating long faces and short ends (Flemish bond or English bond), the walls are likely solid. Our solid wall insulation guide covers options for solid-walled homes.

Check the building age. Homes built after the 1920s probably have cavity walls. Older homes are more likely to have solid walls, and some that do have a cavity have one too narrow to fill. Homes built from the 1990s onwards usually had wall insulation fitted when they were built.

Measure wall thickness. Open a window or door and measure the wall from the inside face to the outside face. The Energy Saving Trust says a brick wall between 260mm and 350mm thick is likely to be a cavity wall, and a narrower one is probably solid. Stone walls are usually solid whatever their depth.

Check your EPC. Your EPC report states whether the walls are cavity or solid and whether they are insulated. Check the GOV.UK EPC register. Use our EPC improvement planner to see how filling the cavity would affect your rating.

If you are still unsure, ask a registered installer for a borescope inspection, where a small camera goes through a drilled hole to see whether the cavity is empty or filled.

The most common concern about cavity wall insulation is whether it causes damp. The short answer: for most UK properties, it does not. But for some properties in specific locations, the risk is real.

Why damp can occur: The original purpose of the cavity was to prevent rainwater from bridging across to the inner wall. Insulation fills this gap. If the external wall is porous, mortar joints are deteriorated, or the property faces prolonged wind-driven rain, moisture can travel through the insulation to the inner wall.

Properties at higher risk: Homes on exposed sites that face driving rain or extreme weather are most at risk. So are homes at risk of flooding, homes with cavities narrower than 50mm or full of rubble, homes with damaged brickwork, pointing or render, and homes with existing damp.

Properties at lower risk: Homes in sheltered locations with a clear cavity of at least 50mm and brickwork, gutters and roofs in good repair. The Energy Saving Trust says these are the homes suited to standard cavity wall insulation.

A competent installer will assess your property's exposure before proceeding. The industry uses a wind-driven rain exposure map to categorise risk. If your property is highly exposed, the installer should either decline the work or propose a fill material certified for that exposure.

If you notice damp or mould after installation, the cause may be the cavity fill, or it may be something else such as poor maintenance made more noticeable. Call the installer first: they should revisit, check the survey and the work, and fix the problem or remove the insulation if the work is at fault. Remedies range from repointing and repairing gutters to extracting the fill, which needs a specialist removal company.

Installation work should come with a 25 year guarantee from the Cavity Insulation Guarantee Agency (CIGA) or an independent insurance backed guarantee. If the installer will not help or no longer exists, check your paperwork for a CIGA certificate: CIGA should then help resolve the problem under the terms of the guarantee. You can check cover on the CIGA website. Make sure any new installation comes with one of these guarantees.

The ECO4 scheme funds cavity wall insulation at no cost for many households receiving qualifying benefits (such as Universal Credit, Pension Credit, Child Benefit or Housing Benefit), in homes rated D to G if owned, or E to G if privately rented. Even if you do not receive benefits, your local council may refer you through ECO4 Flex, or, in England, fund the work through a Warm Homes: Local Grant. The Great British Insulation Scheme closed on 31 March 2026.

At about £2,200 and a measured saving of around £100 a year, the full price case is slow. If a grant covers it, there is no reason not to go ahead, as long as the survey is clear. ECO4 closes on 31 December 2026 and the work has to be finished by then. Use our grant eligibility checker.

If you pay yourself, installing insulation in a home is zero rated for VAT until 31 March 2027, and 5% after that (HMRC VAT Notice 708/6).

Filling empty cavity walls is one of the bigger single gains on an EPC. For landlords, it can help a property reach the minimum EPC rating needed to let it. For homeowners, buyers see the EPC and the lower running costs it describes.

Use our EPC improvement planner to model how cavity wall insulation combines with other upgrades like loft insulation and heating improvements.

If you are planning to install a heat pump, filling cavity walls first reduces your heat demand, which means a smaller heat pump and lower electricity bills. In our model a 3 bed semi needs about a 9 kW heat pump at average insulation and about 7.5 kW when well insulated. At the median price paid under the Boiler Upgrade Scheme that is about £12,700 against £12,300 installed, before the grant.

Combined with loft insulation and draught-proofing, cavity wall insulation prepares your home for the most cost-effective heat pump installation possible. Use our heat pump calculator to see how insulation levels affect sizing and cost.

Once your cavity walls are filled, the next most valuable upgrades are loft insulation (if not already done), draught-proofing, and heating system improvements. A thermal image taken on a cold day can show whether the fill is complete and even.

Read our full guide to improving your EPC rating for the complete priority list of home energy upgrades.

Measured savings from NEED Impact of Measures, England and Wales (DESNZ, 11 June 2026). Cost, suitability and guarantee guidance from the Energy Saving Trust (updated 8 May 2026). Energy prices from the Ofgem price cap, October to December 2026. VAT from HMRC. Guarantee information from CIGA. Heat pump sizes and costs from our heat model.