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.
to continue reading