Extractor Fans and Condensation

One of the major issues in modern homes, which are becoming more highly insulated and sealed against draughts, is condensation.

The Building Regulations relating to mechanical ventilation have now been updated, but a correctly installed and operated extractor fan is still a useful tool.  Having said this however, during our roof void inspections it is sometimes apparent that an incorrectly installed system is venting warm, moist air into the loft, leading to potentially significant damage.

Roof timbers saturated by condensation


The following illustrations show some of the common, and in some cases bizarre, faults that we find during our roof space inspections.


A common fault is the trunking simply falling off the external vent. A very easy repair but one that can prevent significant damage.


In this instance there is no ducting, the fan simply blows warm moist air into the roof.


This fan has no ducting attached but in this case was covered with insulation, preventing the vented air escaping at all.


Excessively long or badly routed ducting can trap condensation, which then drips back into the accommodation below.


In this case the builder had connected the extractor fan into the foul drain.  If the fan wasn’t running the fumes from the drain vented straight back into the bathroom. When the fan was running the smell was blown into the roof space because the top of the pipe wasn’t vented externally either.  Not an inspection to do just before lunch!


And finally; In this case there was a gap in the party wall so the easiest thing to do was push the ducting into the roof of the adjoining house, letting the neighbour deal with the resulting damage near the outlet from the pipe as shown here.

Published 20 December 2022

Written by: Richard Eccleshall BSc MRICS MRPSA