Vapor barrier in temperate climate R40 walls
Hello, I live and build in the midwest with both heating and cooling seasons. When using 2×6 stud construction insulated with cellulose AND two taped layers of 2inch rigid polyisocyanurate foam, where should the vapor barrier go? Fine Homebuilding No.208, January 2010, featured an interesting article titled “6 Proven Ways to Build Energy-Smart Walls”. Example No.5 detailed a method for new construction in which a 2×6 stud wall with cellulose is wrapped with a double layer of polyisocyanurate rigid foam. I like the concept as it is easy for a old school crew to accomplish. My concern is about vapor barriers and condensation, especially for a climate that has both heating and cooling seasons. In a temperate climate zone like the midwest, where should the vapor barrier go? Outside the exterior foam board? Inside just under the drywall? Or somewhere in between? Any discussion of vapor condensation would be much appreciated. Thanks. Stevemachine in Arkansas
Replies
The foam board IS the vapor barrier.
If I'm not mistaken, vapor
If I'm not mistaken, vapor barriers are not recommended in temperate climates, only cold climates.
Frankly, you don't need a
Frankly, you don't need a vapor barrier in Arkansas. More important to do a good job of air sealing, and also flashing and rain screen to the extent necessary with your siding choice.
Steve,
Your plan for an R-40 wall is a good one. Just be sure that your wall does NOT include an interior vapor barrier (like polyethylene).
The foil-faced foam will do an excellent job preventing inward solar vapor drive during the summer. During the winter, the foam will keep your stud spaces warm, eliminating the chance of condensation. As long as you have no interior poly, your walls will be able to dry to the interior if they ever get wet.
Martin Holladay, senior editor
http://www.greenbuildingadvisor.com