Q. Do "vapor-retarder paints" really work? Can I rely on them as the sole vapor barrier in an old house? If so, can you recommend some brand names?

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 with a perm rating of one or less is considered a vapor retarder.

While a vapor-retarder paint doesn’t have a perm rating as low as 6-mil poly (at about .06), it still qualifies as a vapor retarder, and if applied properly, the paint will slow the transmission of vapor through walls and ceilings sufficiently for most situations. Proper application means full coverage of at least one uniform coat (two coats would be better).

Vapor-retarder paints do have some limitations, however. In extremely cold climates, such as you find near Fairbanks, Alaska, or on the Canadian plains, you’ll need to use a better vapor retarder than a coat of paint, especially if the building’s interior is above 50% relative humidity. Also, don’t forget that any vapor retarder only slows the diffusion of vapor — the transfer of moisture through tiny pores in the wall and ceiling materials. You still need a good air barrier to prevent air from leaking through cracks in the building, carrying moisture into wall and ceiling cavities.

J.D. Ned Nisson is president of Energy Design Associates, a New York City-based consulting firm, and editor of Energy Design Update, a monthly newsletter on energy-efficient building design and construction.

Low-Permeability Paints

Aspen Paints 1128 S.W. Spokane St. Seattle, WA 98134 206/682-4603 401 Vapor Guard latex wall primer

Benjamin Moore 51 Chestnut Ridge Rd. Montvale, NJ 07645 201/573-9600 260-00 Moore Craft Vapor Barrier latex wall primer

Fuller O’Brien 395 Oyster Point Blvd. Suite 350 South San Francisco, CA 94080 415/871-6060 220X5373 latex all primer 220-20 latex wall primer

Glidden Paint Co. 925 Euclid Ave. Cleveland, OH 44115 216/344-8000 Insul-Aid latex wall primer

Miller Paints 317 S.E. Grand Ave. Portland, OR 97214 503/233-4491 1545 Vapor-lok latex wall primer

Palmer Industries 10611 Old Annapolis Rd. Frederick, MD 21701 301/898-7848 86001-Seal Vapor Barrier non-toxic modified latex primer

Rodda Paint 6932 S.W. Macadam Portland, OR 97219 503/245-0788 7900 Vapor Block latex wall primer

Sherwin-Williams See Yellow Pages under "Paint-Retail" for a local distributor Vapor Barrier 154-6407 latex wall primer

Note: This list includes common low-permeability latex wall primers suitable for interior use. In addition to these products, just about any alkyd paint or a pigmented shellac sealer can be used to create an effective vapor retarder.