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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 satin-finish one.

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 trap moisture.