Facts last checked 17 September 2026
The short answer
Look at the brickwork and measure the wall. If every brick shows its long side (stretcher bond) and the wall is 260 mm to 350 mm thick at a window or door reveal, it is probably a cavity wall. If you can see short brick ends mixed in, or the wall is thinner than 260 mm, it is probably solid. As a rule, UK homes built before the 1920s have solid walls and later ones have cavities, but render, cladding, timber or steel frames and narrow early cavities all need a professional check before anyone insulates.
Why does it matter which kind of wall you have?
The Energy Saving Trust estimates that about a third of the heat lost from an uninsulated home goes through the walls. How you stop that depends entirely on the wall. A cavity wall is two leaves of masonry with a gap between them, which can usually be filled from outside in a day. A solid wall has no gap, so insulation has to go on the inside or the outside, which costs several times more.
In England, the 2024 English Housing Survey found 54% of homes had cavity or solid wall insulation. Among owner-occupied homes with solid walls, only 11% were insulated. Knowing your wall type is the first step to knowing which of those groups you are in.
What does the brick pattern tell you?
Stand back from an unrendered wall and look at how the bricks are laid. The pattern is called the bond.
- Cavity wall: every brick is laid lengthways, so you only see long faces in a regular, staggered pattern. This is stretcher bond. The outer leaf of a cavity wall is a single row of bricks laid lengthways, so no bricks are turned to run through it.
- Solid wall: some bricks are turned to run through the wall, so you see their short ends (headers) mixed in. Common patterns are English bond, with alternate rows of long faces and short ends, and Flemish bond, with long faces and short ends alternating along each row.
How thick is the wall?
Open a window or an external door and measure from the inside face of the wall to the outside face, including plaster. The Energy Saving Trust uses these guides:
| What you measure | What it probably means |
|---|---|
| Brick wall 260 mm to 350 mm | Cavity wall |
| Brick wall under 260 mm | Solid wall |
| Brick wall thicker than 350 mm | Possibly a brick facing added to another construction, such as a timber frame |
| Stone wall, any thickness (up to about 500 mm) | Usually solid |
| Earth wall, 300 mm to over 1 m | Usually solid |
Thickness is a clue, not proof. Plaster, dry lining or a rebuilt reveal can add or hide centimetres, so use it alongside the brick pattern and the age of the house.
What does the age of your house suggest?
- Built before the 1920s: probably solid brick or stone. Some older houses do have cavities, but the Energy Saving Trust says these are usually too narrow to insulate and should be treated as solid walls.
- Built from the 1920s onwards: probably cavity walls, typically a brick outer leaf and a brick or concrete block inner leaf.
- Built from the 1990s onwards: the Energy Saving Trust says the walls are likely to have been insulated when built.
Our guide to the five UK property types covers what each era usually means for heating and insulation.
What if the walls are rendered, clad or not masonry at all?
Render, pebbledash and tile hanging hide the bond, so you will need to rely on thickness and age, or ask a professional. Some homes are not masonry-walled at all:
- Timber frame: the frame may be visible somewhere, or hidden behind render, cladding or a brick facing. NBS, citing Building Research Establishment guidance, notes retrofitted foam fill is not recommended for timber frame, and the studs stop blown insulation spreading evenly.
- Steel frame and precast concrete homes: the Energy Saving Trust lists these as different wall structures altogether. They need a specialist assessment, not a standard cavity fill.
How do you find out whether a cavity is already filled?
- Check your Energy Performance Certificate. It may record the walls as insulated cavity. Our EPC tool explains what each line means.
- Look for a neat grid of small filled holes in the outside wall, about a metre apart. The Energy Saving Trust describes injection holes of around 22 mm at roughly 1 m intervals.
- Ask your council's building control department whether work was recorded.
- Ask a registered installer for a borescope inspection: a small hole is drilled and a camera shows whether the cavity is empty, filled, or full of rubble.
When is cavity wall insulation the wrong answer?
Most unfilled cavity walls can be insulated, and the Energy Saving Trust says the cost is typically recovered in five years or less. But filling the wrong wall can pull rain across the cavity and cause damp. According to the Energy Saving Trust and the Centre for Sustainable Energy, a wall is not suitable for standard cavity fill if:
- it is exposed to extreme weather or driving rain
- the cavity is narrower than 50 mm, or uneven
- there is builder's rubble or mortar debris in the cavity
- the brickwork or pointing is cracked, porous or poorly maintained, or gutters and downpipes leak
- there is existing damp, or the home is at risk of flooding
What does each option cost, and what does it achieve?
A U-value measures how much heat passes through a square metre of wall for each degree of temperature difference, so lower is better. Approved Document L of the Building Regulations sets the U-values a wall should reach when it is upgraded:
| Measure | Upgrade target U-value | Energy Saving Trust typical cost | Smart Energie typical range |
|---|---|---|---|
| Cavity wall insulation | 0.55 W/m²K | About £2,200 | £800 to £2,200 (three-bedroom semi) |
| Internal wall insulation (solid wall) | 0.30 W/m²K | About £12,000 | £6,500 to £12,000 whole house |
| External wall insulation (solid wall) | 0.30 W/m²K | About £15,000 | £18,000 to £32,000 whole house |
Approved Document L expects a wall that is being substantially renovated to reach these targets, and an existing wall worse than 0.70 W/m²K that is kept in a conversion to be upgraded, where that is technically feasible with a simple payback of 15 years or less. Installation of insulation in homes is zero-rated for VAT until 31 March 2027. If your walls turn out to be solid, read internal vs external solid wall insulation, and see our insulation service page or the retrofit planner for next steps.
Common questions
Can I tell my wall type from inside the house?
Partly. Measure the wall thickness at a window or door reveal: 260 mm to 350 mm for brick suggests a cavity.
My house is from the 1930s. Is it definitely cavity?
Probably, but not definitely. Many homes from the 1920s and 1930s have cavity walls, some have early narrow cavities that are too tight to fill, and some are solid. Check the bond and the thickness, and ask for a borescope inspection before any quote is finalised.
Does cavity wall insulation cause damp?
Not when the wall is suitable and the work is done properly, according to the Energy Saving Trust. Problems come from filling walls exposed to driving rain, narrow or rubble-filled cavities, or masonry in poor repair. If damp appears after installation, go back to the installer first, then to your CIGA or insurance-backed guarantee.
How long does cavity wall insulation take to install?
For an average home with easy access, the Energy Saving Trust says about two hours. The installer drills small holes in the outside wall, blows in insulation such as polystyrene beads, and fills the holes.
I live in a semi or terrace. Does my neighbour have to agree?
No, but the Energy Saving Trust says the installer must fit a cavity barrier where your wall joins the neighbouring house, so the insulation stays on your side. In a flat, the whole block normally has to be insulated, which needs agreement from the building owner.