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Property Clinic: Black mould is forming around our windows. Do they need to be replaced?

I’m removing the fungus with bleach but the problem is getting worse

We’re a young family who moved into our first house late last year. We love our new place, and it was great getting settled into our new home. However, one issue we noticed was that after cold nights our windows have loads of condensation. We would like to avoid having to purchase new windows just now but are there more affordable ways of reducing the levels of condensation? Or would you suggest replacing the windows as the better option?

The outer two corners below windowsill level of our south-facing sunroom are always damp, especially after a very cold night when there is a lot of condensation on the UPVC windows and plastic sealant in the morning. Black mould is forming in the lower corners, which I wipe off with bleach, but the paint and plaster is now coming off. Using a dehumidifier does seem to dry up the corners temporarily. How can I resolve this problem and what can I use to get rid of the mould and repair the wall?

Mould and condensation are common issues in homes throughout the country. Increasingly this is becoming a problem as home occupiers try to seal up their homes from draught ingress.

You mention that condensation forms on your windows, while black mould is forming on lower wall corners. Where moist air is retained in a home it tends to travel to colder parts of the building and result in condensation and mould growth.


The presence of condensation in rooms for prolonged periods of time increases the risk of mould, which is a fungus that is black/brown in colour and provides for an unhealthy environment to reside in. The spores that cause mould to develop are present in the air and require a damp environment to spread.

Moist air generated in a home from bathrooms and cooking activities should be collected at source and removed to the exterior of the building with the assistance of good quality extractor fans. It is important that dwellings are provided with ample ventilation to allow a cross flow of air throughout each room, while all rooms should be provided with heat.

Building defects in the construction of a dwelling can also contribute to the presence of condensation and mould occurring. These can include poorly fitted insulation or lack of adequate ventilation provision in external/ internal building elements, such as walls, floors, and roofs.

While it is difficult to fully diagnose the cause of the condensation and mould in your sunroom without a physical inspection, I recommend you consider the following simple and low-cost steps to reduce the risk of condensation and mould:

  • Ensure all moisture generated within the dwelling is collected and expelled to the outside of the property.
  • Do not dry clothes in the dwelling.
  • Ensure tumble dryers are vented to the outside of the house.
  • Ensure wall vents are present in each room and they remain open.
  • Open windows daily and allow air circulation to occur.
  • Provide room heating especially during cooler times of the year.

While these measures may not fully address your problem, they should assist in reducing the risks of condensation occurring. Construction details incorporated in your property may have to be considered also. Low-cost measures should be tried first and the issue monitored for any improvement.

Ventilation is key to reduce the risk of condensation and mould. It is recommended the affected areas be cleaned down using an anti-fungicidal wash before renewing finishes to remove mould growth.

Andrew O’Gorman is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland

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