2x4 or 2x6?

My intention is to use this blog to discuss questions I am asked all the time. So, the 2x4 vs. 2x6 debate seems like a good opening post.

There are 2 major issues: how much wood is used and the R-value of the wall assembly.

In terms of wood used, if both have the same stud spacing, the 2x6 obviously uses more wood. But if you can go up to 24 o.c. for the 2x6, it's about the same volume of wood used. (About 1/3 less due to spacing, but 1/3 more due to thickness). Some drywall installers complain, but I've seen it work just fine.

In terms of R-value, 2x4 will get you between R-13 and R-15 for most cavity insulations, and 2x6 can give you R-19 to 23. From an insulation standpoint, you're obviously better off with the higher R-value. But it turns out that an R-13 wall with R-5 exterior sheathing is almost exactly equivalent (in terms of overall assembly R-value) to R-20 in a 2x6 wall. So, you could get almost the same performance. In the past, R-5 exterior sheathing has been tricky for a number of reasons: structurally you have use OSB at the corners or use metal bracing, and siding guys aren't crazy about having to locate the studs to nail into. But there is a new product out from DOW that combines structural sheathing with an R-5 all in one product - eliminating all these problems. Of course nobody's saying you can't also use R-5 sheathing on a 2x6 wall...