Climate dictates solutions
Until 2007, the International Residential Code (IRC) treated the country like a single cold climate and offered only one solution to vapor drive. The code required that a vapor retarder be installed on the interior (warm) side of assemblies. However, most of the country has both heating and cooling seasons, so sometimes the cold side is the inside of the wall or roof instead of the outside. The old codes created a risky situation that could cause problems in the summer, especially if the local code official insisted that a vapor retarder meant a polyethylene sheet. When you fill a wall with a highly vapor-permeable insulation (fiberglass batts) and cover one side of it with a virtually nonpermeable vapor retarder (poly), the vapor retarder will be on the wrong side of the assembly for part of the year and inhibit the wall’s ability to dry.
The IRC now breaks the country into eight climate zones and recognizes three classes of vapor retarders that have different levels of permeance (see “What’s the Difference?
#202). Generally, the IRC demands that a class-I or -II vapor retarder be installed on the interior side of homes in climate zones 5 and above, and in marine 4. However, if you’re building in a humid climate in zone 4, 5, or 6, and you air-condition your house in the summer, you may be concerned about having a vapor retarder in the “wrong” position for part of the year. If this is the case, just be sure to use a class-II vapor retarder on the interior of the wall. You also can use closed-cell spray foam in the cavity or a layer of exterior rigid foam, with a class-III vapor retarder on the interior.
When building in hot, humid climates (zones 1 to 3), you shouldn’t have a vapor retarder on the interior side of the wall. This allows any water vapor that makes its way into the wall, which can be tempered by closed-cell foam or exterior rigid foam, to dry to the interior.