Air gaps, insulation and roofing membranes in pitched roofs.
We have a question about what air gap you should leave between the insulation and the roofing membrane in a pitched roof almost every day. Here’s some advice about what to do…
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 shed 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 should follow the guidance in the BS5250 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 moisture to run from the top surface of the membrane. This can be achieved by either:
On a discontinuous surface
- A minimum 10 mm gap between the membrane and tile / slate batten, although 25 mm is typical to maximize thermal performance.
- If your building is built to NHBC standards, then a 50 mm gap is still required.
On a continuous surface (for example, laid flat on the insulation or sarking board)
- An air gap is not required as long as there is a path for moisture to run down the roof.
More detailed standard specifications are shown in the Kingspan Kooltherm K7 Pitched Roof Board brochure.
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