Widely varying performance of vapour permeable underlays ultimately led to the decision by the NHBC and BSI that only air-open membranes could be used without supporting high level ventilation. For roofs needing the highest ventilation performance, Klober Permo® air offers not just an exceptional level of breathability, with a water vapour transmission sd-Value of 2736 g/m2/day,but a water vapour resistance which has been independently confirmed to be as low as 0.03 MNs/g. It has been used to upgrade countless historic buildings and to provide unparalleled performance during the critical drying out period on new builds.