The variable performance of vapour permeable underlays ultimately led to the decision by the NHBC and the BSI that only air-open membranes could be used without supporting high level ventilation. Klober Permo® air offers an exceptional level of breathability, with a water vapour transmission sd-Value of 2736 g/m2/day and a water vapour resistance which has been tested by the BBA and confirmed to be as low as 0.03 MNs/g. As a result, it has been specified for roofing upgrades of many high profile and historic buildings. For new builds, particularly those with complex roof designs, underlay performance is put under greatest strain during the critical drying out period. Permo air’s use on both cold and warm roof installations avoids the additional cost of supporting ventilation and offers the additional benefit of having been tested for wind uplift resistance by the BRE. It found the membrane to be suitable for use up to Zone 5 on the recently introduced BS5534 wind speed map (with laps taped or battened in zones 4 and 5).