Breathable membranes have been used in buildings for many years now and, more recently, in cold
pitched roof constructions without traditional eaves ventilation. The benefit of reduced heat loss and
not having to incorporate ventilators has seen their use grow but at the same time has attracted
noticeable comment both from supporters of the application and from detractors who favour traditional
ventilation solutions.

 

What is a breather membrane and how does it work ?
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.

The function of these membranes is to provide all of the normal demands made on a roof
underlay and at the same time allow water vapour egress without making specific provision
for ventilation.

 

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