Damp and moisture meters
What should you make of an extract fan that blows into the wall cavity?
The moisture is going into the cavity, not outdoors. An extract that discharges into the wall cavity can wet the cavity and the inner leaf. The room has not been ventilated to outside air. A visual survey may see the grille and still not see where the duct ends.
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?
Bathroom and kitchen fans are sometimes pushed into the cavity because that was the shortest route. The cavity is not the outside. Moist air condenses on the cold outer leaf or soaks insulation, and the inner face can then show damp that a meter calls rising damp even though the source is the fan. A grille on the outside wall, with a duct that actually reaches it, is a different installation. The surveyor sees the fan in the room and any external grille they can find. They do not endoscope the cavity as part of a home survey.
The inspection does not test the fan, which is a service looked at visually, not safety-tested and not airflow-tested. If no outside terminal can be found, say that the destination is unknown rather than hoping the cavity is empty. Do not inject a damp-proof course for a patch that lines up with a fan. Ask where the duct goes. If it stops in the cavity, the repair is to duct it outside, then to see whether the wall dries.
What should you ask your surveyor?
- Is there an external grille for this fan, or only the fan indoors?
- Does any internal damp line up with the fan position?
- Are you guessing the duct ends in the cavity, or did you see that?
What are the typical next steps?
- Do not treat a patch beside a fan as rising damp until the duct route is known.
- Look outside for a terminal before you accept that the fan leaves the house.
- If the duct ends in the cavity, rerouting it is the repair. A new course is not.
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
Is a fan through the window pane the same issue?
No. A window kit discharges to outside air, if it is the real outlet and not blocked. It does not fill the cavity.
Does cavity insulation make a fan that blows into the wall more of a problem?
It can hold the moisture the fan dumps. The survey often cannot see that. The missing outside terminal is still the clue.