A.Ned Nisson responds: Vapor-retarder paints definitely work when applied on the warm side of insulated walls, and are especially well suited for existing homes without an installed vapor barrier. The accompanying Source of Supply lists several available brand-name paints.

Vapor-retarder paints typically have a perm rating of .8 to .45, but the actual perm in the field depends on the number of coats and the degree of coverage. A material’s perm rating indicates the ability to diffuse moisture through the material. The lower the perm rating, the better a material’s resistance to moisture diffusion. Any material...

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