Buildings

2x4 or 2x6

Q: One builder we talked with made a big deal about the fact that he uses 2x6 studs instead of 2x4's. He claims this is more energy efficient. Is it, really?

A: The short answer is...maybe. Whether a particular feature contributes to making a house more — or less — energy efficient depends on the entire system being used, not just one piece. Some builders and designers prefer 2x6 construction for several reasons. Most obvious is the fact that the wider studs permit using thicker insulation. A 2x4 stud wall, which is 3 1/2 inches deep, allows 3 1/2 inches of insulation. This could be as high as R-13 or R-15 with the new high-density fiberglass batts or about R-13 with dense-pack cellulose. A 2x6 stud wall, on the other hand, gives you 5 1/2 inches of space for insulation, enabling you to use R-19 or even the new R-22 batts or about R-20 cellulose.

In addition to having more space for insulation, you can use fewer pieces of lumber if you use 2x6's. They can be placed every 24 inches to get the same structural strength as 2x4's placed every 16 inches. Since wood has an R-value of about R-1 per inch, heat will flow through it a lot faster than through the insulation. If you can build a sturdy wall with fewer studs acting as "thermal bridges," you can increase the total R-Value of the wall.

A few builders take this to another step. If you line up your roof rafters or trusses so they fall directly over the studs, you can use a single top plate instead of a double top plate. Taken together, these techniques can reduce the amount of thermal bridges by over 25% and increase the R-value of the ones remaining from R-3.5 to R-5.5.

Another advantage of 2x6 framing is that you have space to insulate the headers with as much as 2 1/2 inches of rigid foam (R-15 to 20), not a significant feature on walls with lots of doors or windows. So, overall, using 2x6 framing offers opportunities to substantially increase the R-value of your walls.

So, what's the down side? Many builders will balk at the added cost. Interestingly, the lumber costs will often come out about the same, the higher costs of the 2x6's being offset by the fact that fewer are needed. Labor costs may be more, particularly with a framing crew inexperienced in 2x6 framing. Most of the additional cost will be in having to cut each stud to size, since pre-cut 2x6 studs are not generally available.

The biggest extra cost will probably be in trimming out around the doors and windows. Some people really like the thicker windowsills, feeling it gives the house a feeling of substance and strength and consider it a major feature of the house. Others consider it clunky and heavy looking, and the added cost is simply not worth it.

To make a fair comparison, you'd have to compare a 2x6 wall with a 2x4 wall of equivalent thermal value. This can, in fact, be achieved using high-density insulation and insulated foam sheathings. This system has the advantage of allowing the framers and trim carpenters to do what they're used to doing.

To get an energy efficient structure, the key is not whether you frame with 2x4's or with 2x6's, but it is instead how the pieces are put together, what kind of framing details are used, and how well the walls are insulated and sealed.