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.

Disclaimer:  

Every care has been taken to ensure that the above content is correct at the time of publication.  Any reliance on the content should be verified by the dutyholder(s) and their professional advisers.  It is the responsibility of the dutyholder(s) and/or person carrying out the work to ensure compliance with the relevant building regulations and associated legislation.

This content is given in good faith by LABC. It is not definitive, and compliance will be a matter for the relevant building control authority to decide as the enforcing body for the building regulations, and ultimately for the appropriate tribunal or court to determine.