Damp and moisture meters
The bathroom duct in the loft is disconnected and the trickle vents downstairs are open. Where is the moist air going, and what fan performance was not measured?
The moist air is going into the loft at the disconnected bathroom duct, and the fan's performance was not measured. An open trickle vent downstairs does not reconnect a duct in the roof. The extract that should leave the bathroom is spilling into the loft space instead. The survey has seen the disconnection. It has not measured the fan, has not timed a flow, and has not decided how much moisture is arriving on the rafters. Open vents on the floor below are a different path. They do not catch the air the broken duct is dumping overhead.
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?
A bathroom fan is only an extract if the duct is continuous to outside. Disconnected in the loft, the fan can still spin and still blow, and the loft takes what should have gone out through the roof or the eaves. Trickle vents admit a little background air to habitable rooms. They are not a termination for a bathroom duct, and being open does not mop up a loft spill. The survey saw a physical gap in the duct.
Ask an electrician or a ventilation fitter to reconnect the duct to a proper outside terminal and to check the fan, which this inspection did not measure. Tell the solicitor the loft is where the moist air is going. Do not accept a note that the trickle vents were open as if that answered the duct. If the loft timbers are already marked, keep that as a consequence to be looked at, not as proof the survey measured the fan.
What should you ask your surveyor?
- Where is the duct disconnected, and where does it point in the loft?
- Did you see a proper terminal on the roof or at the eaves?
- Did you measure fan performance, or only see the open duct?
What are the typical next steps?
- Treat the moist air as discharging into the loft.
- Do not treat open trickle vents as the repair.
- Get the duct reconnected and the fan checked, since performance was not measured.
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 fan is noisy, has its performance been measured?
No. Noise means it was audible. It is not a flow reading. The survey did not measure fan performance. The disconnected duct still spills moist air into the loft whatever the fan sounds like. Audible is not a measured extract, and the loft still takes the air.
Do open trickle vents show the house is adequately ventilated?
No. They show those vents were open. They do not reconnect the bathroom duct, and they were not a performance test. The moist air from that extract is going into the loft, which the open vents downstairs do not stop.