waterproofing went on during (another) rain. we used a higher quality membrane than the building paper typical in
residential construction so that it would double as air barrier / underlayment for the metal roofing. the original
system we designed to - the vmzinc dexter panel - comes with its own underlayment to effectively weep water
away from the zinc, as it is highly corrosive with water. this would have been in addition to a separate air barrier.
when the system was ve'd we wanted a single layer to work both as the vapor retarder and air barrier for the metal
roof - which will have a high thermal absorption, high thermal conductivity, and low reflectivity. although our
weather is exceptionally mild, at 40mm thick, the membrane will help keep the heat on the metal and out of the
interior during the summer.
an excellent overview of underlayments for metal roofing can be found here: metal roofing underlayments
