Is my bathroom fan creating damp on my ceiling?

I have an ongoing issue with damp staining on my bathroom ceiling. My bathroom is on the first floor with an insulated attic above. A damp stain appeared on the ceiling shortly after I inserted an extract fan. The installation of the fan was recommended to me as we had issues with mould in the house. The fan is working well and mould has abated but now this new issue has arisen. Have you any advice on what may be causing the problem? The roof above this area is in very good condition.

The introduction of an extract fan to your bathroom was a good move and clearly has helped reduce the problem of mould. In most homes condensation and mould arises out of water vapour generated through the use of showers and baths. The rapid removal of steam with a fan prevents water droplets forming in the bathroom or escaping and condensing in colder rooms in the house.

The damp stain has only appeared after the introduction of the new fan and therefore, dampness is most likely to be associated with this installation. This narrows the issue down to two distinct possibilities; the first instance, and the more likely one, relates to how the new fan discharges to the outside. It is common to install a vent tile on the roof and connect this with a flexible duct back to the extract fan. The fan, if installed correctly, will be wired with an isolator to the light switch and have a 15 minute over-run. This means that the fan will continue to run after the bathroom light is switched off. However, it is not unusual for this tile to leak or to allow water to enter the duct. If there is a joint or puncture on the duct, this can allow leakage and staining to occur.

Condensing vapour

The second potential issue is the issue of vapour condensing within the duct. Ducts should be insulated to prevent this from happening. Is the duct length excessive? Is the duct uninsulated and located in the “cold zone” above the attic insulation? If so it is common for vapour to condense in the duct. If there is a fall back to the fan location, condensed water can travel backwards in the duct and egress at the fan or at a puncture or poorly formed joint.

My advice would be to first check the connection and fitment of the vent tile or slate and repair the under-tile felt if cut. Check the duct to see if it is punctured. To do this you should pull back the insulation directly above the damp spot on the ceiling to establish if this coincides with a dip or puncture in the duct. If punctured the duct should be replaced. Refit the duct below the insulation so that it is now within the “warm zone” and less likely to be subject to condensation. Ensure that there is a 15 minute over-run on the fan so that all traces of vapour are removed from the bathroom and duct. When these works are completed, you should touch up decoration on the damp patch to the ceiling. If the problem persists, contact your local building surveyor who will assist with a more thorough investigation of the issue.

Noel Larkin is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie