Q. We’re considering installing a layer of 1-inch rigid foam insulation on the underside of a cathedral ceiling framed with scissor trusses. Is the foam a vapor barrier, or will we need to install a poly vapor barrier on the interior side of the foam? Will the foam cause problems when it comes time to finish the drywall?

A.Henri de Marne responds: Extruded polystyrene (such as Styrofoam, Foamular, and Amofoam) and aluminum-faced polyisocyanurate rigid insulation (such as Thermax) make an effective vapor retarder if installed on the bottom chord of any style truss as long as the joints are sealed with a compatible foil-faced tape. If the foil tape is applied carefully at the joints, there should be no need for an additional poly vapor barrier.

By contrast, expanded polystyrene or "beadboard" would not perform as well as a vapor retarder, since this type of rigid insulation can absorb moisture. Assuming that you have R-19 or greater fibrous insulation between the trusses, a 6-mil poly vapor barrier should be installed on the bottom chord of the trusses prior to installing the beadboard.

I don’t see any reason why the foam insulation would cause a problem when installing or finishing the drywall, as long as the drywall is screwed properly to the bottom chord of the truss. Applying 1x3 strapping over the foam will reduce the number of screw misses, however, and creates a dead air space that contributes to a small increase in R-value.

Henri de Marne is a consultant in Waitsfield, Vt., specializing in moisture-related construction problems.