We get lots of questions about what air gap you should leave between the insulation and the roofing membrane in a pitched roof. This article explains how it works.
The gap you leave depends upon the type of membrane used. The membrane acts as a secondary waterproofing layer underneath tiles or slates on a pitched roof it is designed to allow run-off of any water that gets through the tiles or slates.
For refurbishments, where you may have a traditional bitumen-based roofing felt which has high vapour resistance, you show follow the guidance in the BS 5250 Code of Practice for Control of Condensation in Buildings and leave a minimum 50 mm air gap underneath the membrane ventilated to external air. If the membrane is a breather membrane (eg Kingspan nilvent), then there are a number of choices. What is critical is to allow a channel for water drainage to run from the top surface of the membrane. This can be achieved either:
- On a discontinuous surface (for example laid directly over rafters or counter-battens)
- A minimum 10 mm gap between the membrane and tile/slate batten, although 15 mm is typical to maximise thermal performance.
- If your building is built to NHBC standards, then a ventilated 50 mm gap is still required.
- On a continuous surface (for example, laid flat on the insulation or sarking board)
- An air gap in the rafters is not required as long as there is a channel for water drainage to run down the roof, this can be achieved with counter battens that run vertically down the roof.
For more information on pitched roof insulation please see the Kingspan Kooltherm K7 Pitched Roof Board