BS 5250:2002 section 3.26 defines a vapour permeable underlays as 'Pitched roof underlay which meets the resommendations of BS 5534-1 and NFRC TB6 and is addionally water vapour permeable with a vapour reistance of less than 0.25 MNs/g. This figure ca also be expressed as vapour transmission rate of at least 820 g/m2/day when tested under standard conditions of 25 degrees cemntigrade and a humidity of 0 % and 75% rh on each side of the membrane.
