Property clinic: Mould is growing around the windows and in the house

Q. The windows in my house are double glazed with wooden frames and in the last six months there has been a huge amount of mould growing around the windows and in the house. Specifically in my spare bedroom, mould has affected the blinds and books and its appearing in the corner of the room on the walls, even spreading to the contents of drawers and wardrobes. What should I do? The cost of replacing everything will be high so I’d like to fix this so it doesn’t resurface ever again.

A. This appears to be a condensation problem which is due to you and your family’s activity within your home. When the indoor temperature cools down the air cannot hold as much water vapour and it condenses as a liquid becoming visible particularly on cold non-absorbent surfaces such as windows.

The level of condensation will be affected by a combination of four variables: exterior temperature, interior temperature, interior humidity and thermal performance of the windows?

There is nothing you can do about the weather and you can’t stop water vapour condensing on cold surfaces so you have two options: keep the water vapour away from the cold surfaces or stop the surfaces from getting cold.

Improve insulating value by replacing windows with thermally efficient units may help but this is expensive.

Simple steps could include:

• Eliminate window drafts by replacing gaskets.

• Fix the exterior perimeter sealant at the frame reveal juncture.

• Repair defective or loose window handles.

• Check the double-glazed unit has not failed – condensation between the panes will confirm this. Replace the glazing if it has.

In relation to the rest of the room, practical steps can include keeping wall vents clear and avoid covering with a curtain or window blind. Run extraction fans during and 10/15 minutes after the use of a shower and after cooking to expel the moisture before it reaches the windows.

The heating should be on a timer to ensure basic levels of room temperature are maintained. My guess is that the radiator is switched off in the spare room and we are now in the depths of autumn with reduced temperatures externally.

Improving insulation by pumping the external wall cavity from the outside is very feasible and at a reasonable cost now. Check if the reveals are insulated behind the lining/wall plaster. Even with modern double glazed windows, one will often find “cold bridging” at the window reveals and this explains why you are getting the mould growth occurring in the vicinity of the windows.

It is our view that condensation and the resultant mould growth largely arises as a result of a general lack of understanding or awareness on how a house should be lived in and maintained. This is not intended to be a criticism of the occupants or owners of houses but merely an observation based on years of inspecting problems within dwelling houses. If one were to buy a car there would be a fully detailed user’s manual/guide advising on the correct method of servicing and maintaining the car. Perhaps a simple user’s guide on how to live in and control the environment within a dwelling would mitigate the risk of similar problems of this nature arising.

Pat McGovern is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland (scsi.ie)