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.