A.Corresponding editor Henri de Marne responds: Condensation on the underside of steel roofing is a common problem, so your concern is justified. Condensation above a cathedral ceiling can run down the poly vapor retarder until it finds its way through nail perforations in the ceiling, or enter the wall cavity at the base of the ceiling slope.
The best solution is to install the steel roofing over a solid deck covered with a water-shedding membrane like 30-pound asphalt felt. The felt should convey the liquid condensation all the way down to the drip-edge. To provide air flow above the sheathing, it would be preferable to have the roofing elevated above the deck on sleepers and purlins.
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