- 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.