Air permeableBreather membranes first emerged in northern Europe about 15yrs ago and tended to be spunbond polyethylene membranes adhered to a strengthening layer. Following research by the BRE and BSi it was established that a requirement of vapour resistance of below 0.25MNs/g-1 would classify a true breather for use in warm deck and cold non-vented applications. These earlier membranes predominantly had a resistance of 0.23MNs/g-1. and despite problems, such as de-bonding, they established themselves as market leaders. Because the performance of these membranes was just inside this requirement, poor detailing would overload these membranes and incidents of condensation have occurred with aftermarket ventilation then being the only remedy.
Meltblown membranes remain popular and are considered as air permeable (or open air) membranes. Although these membranes allow the good transition of vapour through them, they could not achieve good water tight (head of water) results and the large exchange of air can disperse the water molecules and create 'frosting' on low level wool insulation. They could also be affected by the reverse affect of interstitial condensation.
Permavent membranes are the the only 3 layer membranes to have such a high Moisture Vapour Transfer Rate (MVTR) that they have passed the Air Permiable testing of the BRE. Our very high quality production methods produce membranes that have all the qualities of Air permiable membranes without the problems.
