have been asked to bid on a home in upstate New York that has a
"hot roof." Are moisture problems likely in this unvented roof
A.Moisture problems and heat
buildup are possible in this construction. While
polyisocyanurate or extruded polystyrene insulation is
essentially a vapor retarder, moisture migrating with air
through the cracks in the insulation might pose a problem. To
prevent the possibility of moisture moving into the roof
assembly, install a polyethylene vapor retarder over the
ceiling decking and under the insulation as shown in the
illustration at left. This will serve as both a vapor retarder
and an air barrier.
In addition to potential moisture problems, this type of
roof has no ventilation to cool the back of the sheathing. As a
result, the surface temperature of the roof could get very
high, which might reduce the life of the shingles.