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 cheaper than it looks thanks to grant schemes. The catch is that not every house has a fillable cavity, and a fair few already had theirs filled years ago without the current owner knowing. This guide helps you check, and sets out the real costs, savings and the damp question honestly.
The short answer. Cavity wall insulation typically saves an average semi in the region of 200 to 400 a year on gas, against a cost that grants often bring down to little or nothing. It is one of the best-value single measures available if your house has a clear, fillable cavity in a suitable location. Check first that you actually have an empty cavity, because many homes were already filled and exposed sites are not always suitable.
Does your house even have a cavity?
This is the first question, and getting it wrong wastes money. 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 the different and more expensive approach covered in solid wall insulation.
A few quick checks point you the right way:
- The brick pattern. An unbroken run of long bricks (all stretchers) usually means a cavity. A regular pattern that includes short brick ends (headers) breaking up the rows usually means a solid wall.
- The wall thickness at a reveal. Measure the wall depth at a window or door opening. A cavity wall is noticeably thicker, usually over 260mm, than a single-skin solid wall.
- The age of the house. Pre-1920 is usually solid. Between the wars is a mix. Post-1930 is very likely a cavity, though more modern cavities may already be filled with insulation from new.
If the picture is unclear, a surveyor can drill a small inspection hole and look inside with a borescope. It is worth knowing what you have before you read any further, because the whole job only applies to an empty cavity.
What it costs and what it saves
An empty cavity lets warmth pass straight through the wall. Injecting insulation through small holes drilled in the mortar slows that loss considerably. The material is usually blown mineral wool, polystyrene beads or foam, and the work takes a few hours with no mess indoors.
| House type | Typical full cost | Typical annual saving |
|---|---|---|
| Mid-terrace | 300 to 600 | 120 to 250 |
| Semi-detached | 500 to 900 | 200 to 400 |
| Detached | 700 to 1,400 | 300 to 550 |
At 7.33p per kWh gas, Ofgem cap July to September 2026. Savings are estimates for a gas-heated home and vary with size, exposure and existing insulation. Grants such as ECO can cut the cost to little or nothing for eligible households. Worked examples; check your own situation.
Even at the full price, the payback is among the best of any insulation measure, often two to four years on saving alone. Where you qualify for help through the ECO scheme or a local grant, the cost can fall to little or nothing, which makes it close to a free saving. The holes are made good afterwards and are barely visible once the mortar weathers in.
The damp question, answered honestly
You will hear stories of cavity insulation causing damp, and they are not pure myth. The job is right for most houses but wrong for a few, and being honest about that builds the trust the salesy adverts do not.
The cavity exists partly to stop wind-driven rain crossing from the outer leaf to the inner one. In exposed locations that take a battering of horizontal rain, or where an installation was done badly, moisture has occasionally bridged the gap and shown up as damp patches inside. It is a real, if uncommon, failure mode.
The answer is not to avoid the job, but to use a registered installer who surveys the property first. A proper survey checks the exposure rating of your location, looks for existing damp, cracked render or failed pointing, and confirms the cavity is clean and wide enough. A good installer will decline the houses that should be left alone. Insist on that survey, and make sure the work comes with the standard guarantee so you are covered if anything does go wrong.
It also helps to know the warning signs that your house may not be a good candidate, so you can ask the right questions. A wall that faces the prevailing wind and rain with no shelter, render that is cracked or blown, pointing that has failed, or existing damp patches inside all point to fixing the wall first or leaving the cavity empty. A reputable surveyor will raise these unprompted; if one does not, treat that as a reason to get a second opinion. If your walls already show damp, sort that out before any insulation, and read up on condensation and mould before assuming the cause is the walls rather than ventilation.
Check it is not already full
This is the step people skip and then pay for nothing. Many houses had their cavities filled years ago, by a previous owner, under an old grant scheme, or from new. There is no sense paying to insulate a cavity that already has insulation in it.
Before you commission anything:
- Ask a surveyor to drill a small inspection hole and check for existing fill. A borescope check costs a fraction of the full job.
- Check the deeds, the home information pack, or any past paperwork for a cavity insulation guarantee certificate.
- Look at your EPC, which often records the wall construction and whether it is insulated, though it is not always accurate.
- Ask the previous owner or estate agent if you have just bought the place.
A few pounds of checking saves you the whole cost of an unnecessary job. It also tells you whether a top-up is needed, since some early fills have settled or degraded over the decades.
What gets injected, and what to expect on the day
If your house is suitable and the cavity is empty, the work itself is quick and undramatic. Knowing what happens takes the worry out of it.
The installer drills a grid of small holes, usually around 22mm, through the mortar joints in the outside wall, roughly a metre apart. A hose is fed into each hole and the cavity is filled with one of three materials:
- Blown mineral wool, the most common, which is fire-resistant and water-repellent.
- Polystyrene beads bonded with a light adhesive, which flow easily into awkward cavities and resist water well.
- Injected foam, used less often, which sets in the cavity.
The whole job on an average house takes a few hours, all from outside, with no mess or disruption indoors. When it is done the holes are filled with matching mortar, which is visible at first but weathers in over a season or two until you would struggle to find them. You should receive a guarantee certificate, usually backed by an independent agency, which you keep with the house deeds. A good installer leaves the inside of the house exactly as they found it, because they never come in.
Where cavity insulation sits among your options
Cavity wall insulation is one of the strongest single measures, but it works best as part of a fabric-first plan rather than on its own. The sensible order, best payback first:
- Draught-proofing everywhere, the cheapest win of all.
- Loft insulation topped up to current depth.
- Cavity wall insulation if you have a fillable, suitable cavity. This page.
- Floor insulation and double glazing, slower paybacks bought partly for comfort.
- Solid wall insulation for older homes with no cavity, the biggest job.
Once the fabric holds heat better, the heating works less hard, so it pairs naturally with sensible thermostat settings and dropping the boiler flow temperature on a condensing boiler. A well-insulated house is also the right starting point if you are ever weighing up a heat pump against a gas boiler, since heat pumps work best in homes that hold their warmth.