A1 The particular membranes in question typically comprise spunbonded polypropylene or spunbonded polypropylene/polyethylene laminated either side of a micro-porous polypropylene or polyethylene film.
Monolithic spunbonded membranes are also used. They have a structure that is sufficiently fine to prevent liquid water penetration in service conditions, but not too fine to prevent the transfer of water vapour (molecules of water in air), see figure 1.
All materials, including vapour control layers, have a finite permeability to water vapour transfer. However, in the context of this Article, a breathable membrane is a material which, in service conditions, is sufficiently permeable to water vapour transfer to adequately limit the risk of condensation in roofs.
Current published definitions include the following:
BS 5250: 2002 section 3.2 “Breather membrane” – vapour permeable membrane with a vapour resistance less than 0.6 MNs/g. The Standard goes on to describe the use of ‘Breather membranes’ in walls.
BS 5250: 2002 section 3.26 “Vapour permeable underlay” – Pitched roof underlay which meets the recommendations of BS 5534-1 and NFRC TB6 and is additionally water vapour permeable with a vapour resistance of less than 0.25 MNs/g
These two figures can also be expressed as water vapour transmission rates of at least 341 or 820 g/m2/day respectively, when tested under standard conditions of 25o C and humidity of 0% rh and 75% rh on each side of the membrane.

Links: breathable membrane, roofing membrane, roofing foil, membrane, fiberglass, valve, floor drain, pig drinker,link1, link2