What are foil breathable membranes and when should they be used?
Foil breather membranes are breather membranes with a low emissivity foil face.
Emissivity is the value given to materials based on the ratio of heat emitted compared to a black body on a scale of 0 to 1. A black body would have an emissivity of 1 and a perfect reflector would have a value of 0 – so, the lower the emissivity the better.
Foil breather membranes are typically used in wall applications to minimise the thickness of the construction. They will give a small thermal performance increase when used next to a ventilated cavity (eg behind cladding), but have a more significant effect when used next to a sealed airspace (eg in the cavity between masonry and timber frame).
In the best cases, the use of a foil breather membrane could reduce the amount of insulation needed by between 10-20mm (but typically 5-10mm), although this varies with construction type and target U-value.
See the Kingspan Kooltherm K12 Framing Board brochure for tables of the effects of different breather membranes.
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