Our south-facing sunroom is damp and mouldy. What can we do?

Property Clinic: Proper ventilation is crucial when it comes to tackling condensation

The outer two corners below the window sill 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, and the paint and plaster is 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 Ireland. 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 leads to increased chances of mould growth forming. Mould is a fungus and is black or brown in colour. The spores that cause the mould to develop are present in the air and they require a damp environment to spread.


The presence of mould (fungus) provides for an unhealthy environment to reside in.


Moist air generated in a home from bathrooms and cooking activities should be collected at source and be 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 rooms should be provided with heat.

The presence of condensation and mould occurring in a dwelling’s construction can also be contributed to due to building defects incorporated in the construction. These include the external building elements, such as walls, floors and roofs incorporating no or poorly fitted insulation and the lack of ventilation provisions.

While it is difficult to fully diagnose the cause of the condensation and mould in your sunroom, I recommend the following simple and low-cost steps be considered to reduce the risk of condensation and mould:

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

While these simple measures may not fully repair your problem, they will in my opinion 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 firstly considered, and the issue be monitored.

Ventilation is key in reducing the risk of condensation and mould. To remove mould growth, it is recommended that the affected areas be cleaned down using an anti-fungicidal wash prior to renewing finishes. – Andrew O'Gorman

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