The walls of a home typically represent the largest energy losses of any insulated assembly and, therefore, offer the largest opportunities for gains. Because the walls represent the largest surface area, they are most often the thinnest insulated assembly, in addition to having windows and doors in them. Since we’re unlikely to dramatically change the percentage of surface area between walls, floors, and ceilings, we should instead focus our energy on the things we can change: wall thickness and window and door performance.
In many parts of the U.S., a standard framed wall incorporates 2x6 framing with double top plates, studs 16-inches on-center, with R-21 fiberglass batts installed in the 5.5-inch-deep cavity. Intermediate framing assumes space for insulation in window headers, 2 or 3 stud corners, and ladder framing at interior to exterior wall intersections to eliminate uninsulated cold spots where insulation would be burdensome to install. This assembly represents an 18% “framing factor” (the percentage of a wall assembly that is comprised of framing members instead of insulation), which leaves 82% of the wall to be filled with insulation. Since the wood framing members have a thermal resistance (R-value) roughly equal to only one-quarter the R-value of typical insulation products, the overall R-value of the wall is calculated at 18.2. Builders who opt for a higher quality install of fiberglass insulation through a blown-in application can assume R-23 cavity insulation, but with the 18% framing factor included the overall wall R-value is calculated at 19.6.
Shutting Down Thermal Bridging
Conductive energy loss through wood framing is commonly referred to as thermal bridging. The wood framing acts as a conduit for heat energy to travel freely from inside to outside in the heating season and from outside to inside in the cooling season. This can be seen though infrared cameras or with the naked eye on cold mornings where dry stud lines are evident every 16 inches between dew on frost-covered exterior claddings. To save money on lumber costs and increase the percentage of wall insulation over wood framing, some builders have adopted advanced framing techniques, which increase stud spacing from 16 to to 24 inches o.c. and usually calls for single top plates. (However, with this technique, rafters or trusses must sit perfectly on top of each wall stud, so it’s fairly uncommon to build with single top plates in 24-inch o.c. framing).
Advanced framing techniques only increases the overall R-value of walls by roughly 5%. Therefore, thicker wall assemblies that reduce thermal bridging are the answer to gaining superior performance. Double stud or staggered stud walls with two 2x4 framed walls set apart by an inch or more allow for additional insulation, with very little thermal bridging from inside to outside.
An increasingly common approach is to frame exterior walls with standard 2x6 framing and then add 1 inch or more of exterior insulation to the outside of the wall (see images below). This is most commonly rigid foam insulation on the exterior of the sheathing. Alternatively, the rigid foam insulation can be sandwiched between the wall studs and the sheathing (with an increased nailing schedule). For builders interested in pursuing this approach, premade products can help by eliminating a few steps. For builders concerned about the permeability of their walls or the negative environmental impacts of many foam products, mineral wool panels provide another option for continuous exterior insulation to eliminate thermal bridging.
There are many approaches to providing a thicker wall that eliminates the thermal bridging, and every one of them will produce slightly different results when it comes to the overall R-value of each assembly. For the purposes of illustration, let’s compare the overall R-value of 19.6 for the 2x6, intermediate framed wall with blown-in fiberglass insulation to the same wall with one inch of polyisocyanurate (polyiso) foam with an R-value of 6 applied continuously to the exterior. This approach provides an overall wall R-value of 27.
Thermal Impact of Windows
A whole R-value of 27 sounds great, but now let’s consider the effect on the overall wall R-value when adding in windows. Thermal resistance of windows is rated in U-factor. Modestly efficient double-pane windows with a U-factor of 0.30 equate to only R-3.3; efficient double-pane windows with a U-factor of 0.25 equate to R-4; and very efficient triple-pane windows with a U-factor of 0.20 equate to R-5. (Note: higher numbers are better in R-value; lower numbers are better in U-factor). If we incorporate an 18% window-to-wall ratio into our whole-wall R-value, the window wall with a U-factor of 0.30 brings the whole-wall R-value of 27 down to 11.9. A better performing window with a U-factor of 0.25 brings the whole-wall R-value up to 13.3, and a very efficient triple pane window with a U-factor of 0.20 brings the whole R-value up to 15.1. If we install these same three windows packages to the 2x6 wall blown-in with no exterior insulation, we find that the whole-wall R-value ranges from 10.4 to 11.5 to 12.9, respectively.
This sobering effect of a diminished whole-wall R-value is even more evident when uninsulated, solid wood doors are incorporated in the exterior walls of any home’s design. Always consider installing fiberglass or steel exterior doors that offer insulated foam cores, which provide a total door R-value of 5 to 7 over a typical solid wood door with R-values under 2 (substantially lower than most new double-pane windows).
Bottom Line: Whole-wall R values are often dramatically less than one might assume. Reducing thermal bridging with advanced wall assemblies can help to address this issue. Windows and doors represent the biggest negative impact on whole-wall R value,s therefore high-performance windows and doors should always be specified. A good design strategy for a project on a tighter budget would be to eliminate as many windows and doors as possible.
