Damp and moisture meters
The bathroom extractor duct ends in the loft insulation and does not reach the outside. Where is the moist air going?
The moist air is going into the loft, where the duct ends in the insulation. The bathroom extractor discharges into the quilt instead of to the outside, so vapour from showers is put into the roof space. The survey has seen that the duct stops short. It has not measured the airflow, and it has not found an outside terminal joined to this run. A fan that stops in the loft is not extracting to the open air. The moisture stays in the roof.
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?
An extractor only helps the bathroom if the duct carries the wet air out through a roof, a soffit or a wall terminal. Ending in the insulation dumps that air where it can condense on felt, timber or the underside of the covering, and wet the quilt. The fan may still sound as if it works. Noise is not a route to outside. The survey saw the unfinished end in the loft. It did not run a flow test, and it did not redesign the duct.
Ask an electrician or a ventilation fitter to extend the duct to a proper outside terminal, as work beyond this report. Tell the solicitor the bathroom fan discharges into the loft insulation. That is a moisture path into the roof, not a note that the fan is missing. Do not tape the duct and call it finished without an outside end. The survey has not measured a ventilation rate, and it has not said the loft is already rotten. It has said where the air goes.
What should you ask your surveyor?
- Where does the bathroom duct end, relative to the outside?
- Is the moist air discharging into the loft insulation?
- Was any airflow left unmeasured?
What are the typical next steps?
- Treat the duct as dumping moist air into the loft.
- Ask a fitter to carry it to an outside terminal.
- Do not read a noisy fan as extraction to the open air.
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
If the bathroom window opens, does the duct not matter?
A window can air the room when someone opens it. It does not change where this duct blows. The fan still ends in the loft insulation, so when it runs the moist air goes into the roof space. The survey has not measured a rate, and it has not found an outside terminal on this duct. The window is a separate opening.
Has the survey tested the fan motor?
No. Seeing where the duct ends is not a test of the motor. The survey did not measure airflow and did not report an electrical test. The point is the route: moist air is going into the insulation because the duct does not reach outside. A working motor would still be blowing into the loft until the duct is extended.