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.
In your case, you are working from below after the roofing
is installed, so the next best solution would be to have
urethane foam insulation sprayed between the rafters. Since the
insulation would seal all convective paths from the heated
interior, it would effectively prevent condensation. The
disadvantage of sprayed urethane is its relatively high
It is possible to insulate from below, as you suggest, with
rigid polystyrene insulation. First, nail 1x2s to the rafter
sides, up against the roof strapping. This will provide an air
space above the foam insulation, to improve ventilation on the
underside of the metal roofing. Without an air space under the
strapping to promote drying, condensation can cause the
strapping to rot. Install the rigid insulation in layers,
staggering the joints as much as possible. To prevent warm
humid air from rising or condensation from trickling down, you
must caulk each panel to the next, and to the rafters, as they
are installed. Since rigid insulation shrinks slightly with
time, use a quality polyurethane caulk.
Whatever insulation method you choose, you should install
6-mil poly on the warm side.