How to avoid condensation

This is what happens where there isn’t enough ventilation - the roof covering offers very good waterproofing, preventing rainwater from getting into the building. But it also prevents water vapour inside the building from escaping.

Timbers which have rotted through due to lack of ventilation. They in turn cause condensation.

In this case, insulation was fitted between timber rafters, creating an unventilated space between the insulation and the underside of the roof covering. As a result, moist air from within the building has been condensing on the underside of the cold external roof covering.

Why timbers rot and cause condensation

• The void isn’t ventilated, which means water vapour and condensation can’t escape and so ends up being absorbed by the timber structure, causing it to rot.

How to avoid condensation caused by rotting timbers

• When insulating existing roofs, ensure there’s adequate ventilation of the space above the insulation.
• Remember to provide a suitable vapour check layer on the warm side of the insulation.

Remember, while insulation will improve the thermal efficiency of the building and result in lower fuel bills for the homeowner, doing it incorrectly can lead to problems such as condensation, mould growth, poor internal air quality and damage to the building structure.

For help and advice always contact your local building control team.

^{Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.}

^{This article was reviewed and updated on 10 August 2023}