Furring in your exterior walls
While a standard 2×6 wall is a good choice in many regions of the country, it doesn’t quite cut it in Interior Alaska, where heating fuel is expensive and winter temperatures dive to 50-below-zero. That’s why many homeowners in Fairbanks go way beyond the standard, building 24-inch-thick insulated walls or adding 8 inches of foam to their existing walls. But what if you’re in the middle–you want more than a standard stud wall but aren’t quite ready to invest in a super-insulated home?
“Furring in” has become a popular building technique for just this reason. By adding an inch or two of foam on the inside of your walls to reduce heat loss through the studs, it’s a fairly quick, straightforward process that can save a lot of energy. Here’s a rundown of the system.
You can start with any kind of frame wall, though 2×6 or 2×8 studs make more sense than 2×4 walls in a climate like Fairbanks. Good air sealing is critical. The standard practice in Alaska’s severe climate is to wrap the inside of the stud wall with a layer of 6-mil polyethylene plastic sheeting, which blocks both air and vapor from inside the home from moving through the wall in winter time. Anywhere the sheets overlap, acoustical sealant should be applied to a place with solid wood backing. While some holes in the vapor retarder are unavoidable due to wiring and plumbing penetrations, they can be minimized by planning ahead.
Now it’s time to “fur in” the wall, which entails installing strips of wood over the vapor retarder. You can use many different sizes, but 1.5 x 1.5 inch strips are common because they provide room for electrical boxes to fit in the space. Furring strips usually run horizontally as this minimizes the contact between the strips and the studs, which thereby limits heat from conducting through the wall.
Following the ratios
Foam board is then cut and friction-fit between the furring strips. Many homeowners use 1.5 inches of foam, providing an R-value of about 6.5. The key to adding interior insulation is to follow the warm-side-of-vapor-retarder insulation ratios that work in your particular climate. In Interior Alaska, no more than one-third the total wall R-value should be on the warm side of the plastic sheeting. If you use more, it can cool the sheeting to the point where it attracts condensation (assuming warm air is able to get past the foam board and contact the plastic). From here, drywall is installed on the interior of the foam.
Information on how to select and orient furring and drywall can be found from the U.S. Gypsum Council Construction Handbook.