Wall ties and cavity insulation
Why does the Energy Saving Trust treat a cavity narrower than 50mm as unsuitable to fill?
The Trust says cavity wall insulation suits unfilled cavities that are at least 50mm wide and clear of rubble, with sound masonry, and not on walls exposed to driving rain or flooding. Narrower cavities, often in older houses, are unsuitable. A surveyor’s guess at the width is not that measurement.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
The Trust also says exterior repairs should be done first: damaged brickwork or pointing, roofs and overhangs, gutters and downpipes. Those jobs are about keeping rain out, not about the ties. A home survey may say the cavity was not measured. Measuring it means drilling or looking in with a borescope, which is outside a standard visual inspection unless you agreed it.
Do not treat a surveyor’s phrase “cavity wall” as permission to fill it, and do not treat the Trust’s suitability list as a diagnosis of tie failure. If the report has flagged ties, damp or a narrow cavity, send that page to the installer and ask them to confirm suitability in writing. This page does not quote a price. The Trust’s own page carries its current cost notes, and they are not your quote.
What should you ask your surveyor?
- Did you measure the cavity, or are you only saying the wall is cavity construction?
- Is the wall exposed to driving rain, or in good masonry condition?
- What repairs would the Trust’s list say to do before any fill?
What are the typical next steps?
- Get the installer’s suitability check in writing if you still want fill after you buy.
- Do not fill a wall the survey has marked for damp or ties until that cause is dealt with.
- Keep the Trust’s advice as a reason to ask questions, not as a specification for this house.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Does 50mm include the mortar snots?
The Trust says the cavity should be clear of rubble. Debris bridging the cavity is a reason not to treat it as a clean 50mm void. A visual survey may not have seen that debris.
Is every pre-1920 wall too narrow?
The Trust says cavities under 50mm are usually in homes built before the 1920s. “Usually” is not a measurement of this wall. Ask for it to be measured if fill is the plan.