Wall ties and cavity insulation
Polystyrene dust lies on the path under a line of filled holes. Does the dust prove the cavity is fully filled?
No. The dust does not prove the cavity is fully filled. Polystyrene dust under a line of filled holes shows that bead material was handled at those holes. It does not show that every part of the cavity received it. Voids over windows, at reveals and in corners can stay empty while dust falls outside. The survey saw debris and filled holes. It did not open the cavity and has not mapped the fill. A full fill is unproved.
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?
Bead insulation is blown through holes that are then made good. Dust at the foot of the wall is a by-product of drilling or blowing. It is not a measure of what stayed inside the leaf. Missed bays and thin patches are invisible from the path. The holes show someone drilled a line. The dust shows material was at the face. Neither is a completeness test. The survey's limit is that it did not look inside the cavity.
Tell the solicitor there is dust and a line of filled holes, and that a full fill was not proved. Ask for any insulation guarantee and read which walls it names. A specialist can inspect the cavity if you need to know whether it is filled. The survey has not done that and has not said the fill affects the ties. Do not tell a lender the walls are fully insulated because of dust on the path.
What should you ask your surveyor?
- Which walls have the filled holes, and which do not?
- Is there a guarantee that names this insulation?
- Was the cavity opened, or only seen from the path?
What are the typical next steps?
- Describe dust and filled holes, not a proved full fill.
- Ask the solicitor for any insulation guarantee and read which walls it names.
- Do not tell the lender the cavity was fully filled.
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
Do the filled holes show the wall ties were replaced?
No. Filled holes and polystyrene dust point to bead insulation, not to a tie replacement. The survey did not see new ties and did not test the ties. Insulation work and tie work are different and can be done apart. Dust on the path does not identify the ties, and it does not prove the cavity is fully filled either.
Is the dust on the path a sample from inside the wall?
No. Dust that fell outside is not a sample taken from the cavity, and the survey did not send it for analysis. It shows material at the face under the holes. It cannot show gaps or whether beads remain in the wall. The cavity was not opened. The dust is debris, not a test of the fill.