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.

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.

The problem

  • 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.

The solution

  • 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, whilst 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.

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