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Breather 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.
The early use of Polypropylene membranes involved running the membrane over a spiked roller and although this system is undeveloped, these membranes also remain popular 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. They could also be affected by the reverse affect of interstitial condensation and some users were concerned that the holes would be blocked by slate debris.
