Many green builders like double-stud walls. Double-stud walls use affordable and environmentally appropriate materials to achieve a high R-value.
The classic double-stud wall is made up of two parallel 2×4 walls with a space between them. If the framers leave a 5-inch space between the two rows of studs, this type of wall provides room for 12 inches of insulation — for example, dense-packed cellulose, blown-in fiberglass, or mineral wool.
There are variations on this theme; for example, the wall can be thinner (9 inches is possible) or thicker (perhaps 16 inches). Instead of two rows of 2x4s, some double-stud walls have a 2×6 structural wall paired with a parallel 2×4 wall.
Every now and then, an enthusiastic builder will suggest an “improvement” to the double-stud wall: namely, a layer of exterior rigid foam insulation (see Image #2, below). While this layer of exterior foam might (intuitively) seem like a good idea, in fact it is usually a very bad idea.
Encouraging a wall to dry to the exterior
Traditionally, cold-climate walls have been designed to dry to the exterior. In a cold climate, the moisture drive during the winter is from the interior of the house toward the exterior. The interior air is warm and moist, while the exterior air is cool and dry. Under these conditions, moisture will move by diffusion from the interior toward the exterior. (For more information on this process, see All About Vapor Diffusion.)
If the double-stud wall has plywood or OSB sheathing on the exterior and painted drywall on the interior, moisture tends to diffuse through the drywall and the wall insulation toward the exterior sheathing during the winter. Because the exterior sheathing will be cold, it will absorb moisture; in some conditions, a layer of frost may even build up on the sheathing.
If the wall…
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