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...
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