Cavity, solid, or already filled?
Which insulation you can have is decided by the wall, not by the budget. There are three answers, you can usually narrow it to one from outside the house, and it changes the price of the job by thousands.
People ring us about external wall insulation who have an unfilled cavity, which is a far cheaper job, and about cavity insulation on houses that have none. Establishing the wall first saves both conversations.
The short version
A house built between roughly the 1930s and the 1990s almost certainly has a cavity. One built before about 1920 almost certainly does not. Anything from about 2000 onwards was built with the cavity already insulated. Between those dates it is worth checking rather than assuming.
Reading the brickwork
A solid wall is usually built in a bond that includes headers — bricks laid end-on, so you see the short face. They tie the two leaves together, which is what makes the wall solid. Look for a repeating pattern where some bricks are obviously shorter than their neighbours.
A cavity wall is normally built in stretcher bond: every brick shows its long face, course after course, because there is nothing to tie across — the two leaves are held by wall ties inside the gap instead.
Rendered, pebbledashed or clad walls hide the bond entirely. On those, the thickness measurement below is the reliable test.
Measuring at a door or window
Open an external door and measure the depth of the reveal from outside face to inside face, allowing for the plaster. Around 260mm or less usually means solid brick. Around 300mm or more usually means a cavity. It is not conclusive on its own — stone, block and modern construction all vary — but combined with the age and the bond it is usually enough to know before anybody visits.
Has it already been filled?
A previously filled cavity often shows small drilled holes in a regular grid, usually 22mm and made good with mortar that does not quite match. They are easiest to spot on a sunny day at a shallow angle. Filling dates matter too: a cavity filled decades ago may have settled or been done with a material that has since failed, and that is a different job from an empty one.
A borescope inspection settles it properly. A small hole is drilled, a camera goes in, and you see what is actually in the cavity rather than what the paperwork says. That is part of our survey.
What each answer means for the work
| What you have | What fits | Roughly what it involves |
|---|---|---|
| An empty cavity | Cavity wall insulation | Drilled and filled from outside, usually a day, no disruption inside |
| A solid wall, and you can change the outside | External wall insulation | Insulation and render applied outside; scaffolding; the elevation changes |
| A solid wall, and you cannot change the outside | Internal wall insulation | Board applied inside, room by room; takes depth off the room; redecoration after |
| A filled cavity that has failed | Extraction and refill | The old material comes out before anything new goes in |
Why it is worth getting right
The three measures are not interchangeable and they do not cost the same. Fitting the wrong one, or filling a cavity that should not be filled, causes damp rather than fixing heat loss. The survey exists to settle which wall you have before anybody prices the work.
Sources. Construction periods and wall-bond patterns are general building conventions rather than a published standard, and are indicative. Wall thickness varies with materials and with later alteration. The only conclusive test on a specific property is inspection, which is what the survey is for.