February, 2007

Measuring the Costs and Benefits of 2x6 Walls

By Steve Easley

Q:A Columbus, Ohio builder asks, “Given the additional framing costs, do 2x6 walls make economic sense?” 

A:This is a question I often get asked. In residential construction, the issue has more to do with energy savings—since you can pack more insulation into a deeper wall cavity —than it has to do with structure. To arrive at a good answer, the builder needs to weigh the added lumber cost versus the potential savings in the home’s energy bill. This answer will depend on local energy codes and energy costs.

In general, I’m a strong believer in building energy-efficient homes, and advocate that consumers make their homes as energy-efficient as they can afford. However, there comes a point when adding insulation is not the most cost-effective measure you can take to save energy.

Installing high-performance windows and air barriers in exterior walls, and sealing the HVAC ductwork are a few of the things that will go a long way towards improving the energy performance of a home, but will be far less expensive and more effective than the savings you get from 2x6 walls.

Calculating Payback

The most common insulation used in 2x6 is walls is R-19 batts. An R-19 batt is 6 1/4 in. thick, yet we all know that the depth of a 2x6 is 5 1/2 in. When you compress a 6 1/4-in. batt into a 5 1/2-in. cavity, the R-value drops to about R-17. So in reality, the difference in R-value between an R-13-in. batt (for a 2x4 wall) and the standard batt for a 2x6 wall is only R-4.

While costs vary, the additional framing and insulation costs for 2x6 walls would be about $1,500. Using the higher level of insulation, the energy savings for a 2,000 sq. ft. home in a Midwestern climate would be less than $100 per year. This means those 2x6 walls could have a 15-year payback. (And this doesn’t include the additional costs for installing jamb extensions in thicker walls, built up headers, and so on. Those costs could be substantial, depending on the windows selected.)

On-Center Spacing

If you space the 2x6s at 24 in. on center, there are some lumber savings, but a builder may also need to increase the drywall on exterior walls to 5/8-in. panels. (Because the gypsum board is now spanning 21 1/2 in. instead of 14 1/2 in., some people believe thicker panels are needed in large rooms or the walls will look wavy.) Many buyers also get irritated with 24-in. on-center framing when they try to hang towel bars and pictures, for example. And there is a loss of usable square footage: A 28x40, two-story house would lose about 45 sq. ft. of usable floor area with the larger 2x6 framing.

Recommendations

If you are going to frame with 2x6s and are using fiberglass insulation, select higher-density R-21 batts. If you frame with 2x4s, select high-density R-15 batts. These high-performance batts cost slightly more but you get a bigger bang for the buck.

Another option for a high performance wall is closed cell spray foam insulation, with an R-value of 6.9 per inch.

And remember: No matter what type of insulation you select, it is critical to install it correctly. Poorly installed insulation can perform at half its value.