Before you spend a penny on wall insulation, you need to know one thing: is your wall solid or cavity? It decides which measures are possible, what they cost and how disruptive they are — and it's the most common thing homeowners get wrong. Here's how to tell, in a few minutes, without specialist kit.
Why it matters so much
A cavity wall has a gap between two masonry leaves that can often be filled with insulation relatively cheaply and quickly. A solid wall has no gap, so it loses far more heat and can only be insulated by adding a layer to the outside (EWI) or the inside (IWI) — a bigger investment. Getting this wrong wastes money, so it's always confirmed before any quote.
How to tell which you have
1. The age of the house
Pre-1920 homes are almost always solid wall. Homes from the 1930s onwards are usually cavity. The 1920s is a genuine mix — don't assume.
2. The brick pattern
Look at the exposed brickwork. If every brick shows its long side (a uniform “stretcher” pattern), it's likely a cavity wall. If you see a regular pattern that includes the short ends of bricks (“headers”), that's the tell-tale sign of a solid wall, where bricks are laid across the wall to bond it.
3. The wall thickness
Measure the wall depth at a window or door reveal. Less than about 26cm (excluding plaster) points to a solid wall; more than about 26cm suggests a cavity. Rendered or pebbledashed walls hide the brick pattern, so thickness and age are your main clues.
4. A borescope check (the certain one)
The definitive method is a small drill hole and borescope to look inside the wall — something we do as part of a free survey. It also reveals whether a cavity is clear, already insulated, or too narrow to fill.
Cavity vs solid: side-by-side
| Factor | Cavity wall | Solid wall |
|---|---|---|
| Typical age | 1930s onwards (some 1920s) | Pre-1920s |
| Brick pattern | All stretchers (long sides) | Mixed headers & stretchers |
| Wall thickness | Over ~26cm | Under ~26cm |
| Heat loss | Lower | High (30–40% of the home) |
| Insulation route | Cavity fill if suitable | External or internal wall insulation |
| Relative cost | Lower | Higher |
Insulation options for each
Cavity wall: if the cavity is clear, dry and wide enough, filling it is excellent value. If it's unsuitable or already filled, EWI or IWI are the fallback.
Solid wall: your choice is external or internal wall insulation — compared in full in our EWI vs IWI guide. Period homes with solid walls also have their own guides: Victorian terraces and Edwardian houses.
Cavity vs solid wall FAQs
How can I tell if I have a cavity or solid wall?
Three quick checks: age (pre-1920 is almost always solid, post-1930 usually cavity, with the 1920s a mix), brick pattern (a regular stretcher pattern of long bricks suggests a cavity; a pattern that mixes long and short brick ends — headers — suggests solid), and wall thickness at a window or door reveal (under about 26cm tends to be solid, over about 26cm cavity). A borescope check confirms it for certain.
Why does it matter whether my wall is cavity or solid?
It determines which insulation is even possible. A suitable cavity can often be filled relatively cheaply. A solid wall has no gap to fill, so it must be insulated externally or internally — a bigger, more involved job. Specifying the wrong measure is the most expensive mistake in wall insulation.
What year did cavity walls become standard?
Cavity walls became increasingly common through the 1920s and were the norm from around the 1930s onwards. Homes built before about 1920 are almost all solid wall; there is a genuine mix during the 1920s, which is why a physical check matters for that era.
Can a house have both cavity and solid walls?
Yes. Extensions, rebuilt rear walls, bay windows and porches can differ from the main structure, so it is not safe to assume the whole house is one or the other. A survey checks each elevation.