- Q.What is the best way
to insulate and vapor-proof an existing plaster wall without
demolishing the surface?
A.It is possible to insulate an existing
wall with one of the blown-in insulation materials,such as
cellulose, rock wool, or blowing-grade fiberglass. The wall
cavities should be blown tightly to prevent settling.
An existing wall cannot be made totally vapor-proof, but
with enough paint you can prevent any damage from moisture
diffusion if indoor humidity levels are kept to reasonably low
levels (no condensation on double-glazed windows). Most plaster
walls are old enough to have been painted with two or more
coats of an oil-based paint. If not, then two coats of
oil-based paint should be applied. An enamel is best, even a
The most important step is to seal all openings on the warm
side as tightly as possible. This means applying foam sealant
around all outlets and other penetrations of the inside
surface. Seal the bottom of the wall where it meets the floor,
either with a thin bead of caulk or construction tape.
The outside of the wall should not be vapor sealed. The
vapor permeability of the outside finish should be at least
five times greater than the interior finish. A housewrap, such
as Tyvek or #15 felt, which will slow incoming air, will not