- Q.Can rigid foam board be used
on the inside of a wall as both insulation and a vapor
retarder? If so, how should it be detailed?
A.I think the inside
of the exterior wall is the best place to install
foam insulation board. If it is to be used as a
vapor retarder, it must be an extruded foam or a
foil-covered foam board. The joints and any holes
in the surface must be sealed with tape, just as
you would with a polyethylene vapor retarder. The
joint between the floor and the foam can be sealed
with caulk or the gasketing used for airtight
drywall installations. The simplest approach is to
apply foam board over the inside face of the studs
after the wall cavity is filled with insulation,
and then screw the drywall to the studs through the
foam, using longer drywall screws.
I know of several houses built this way more
than 10 years ago, and they have performed
exceptionally well.