I am debating whether to use 2x4 exterior walls with DOW structural insulated sheathing (R-15 for the 2x4 cavity + R-3 for the DOW SIS), or 2x6 exterior walls with R-21 blown in cellulose.
I know the 2x6 option would have a higher R-value, but would also have more thermal bridging.
The 2x4 option would also not need additional sheathing and house wrap.
What would your recommendation be?
As you point out, there are many factors that go into deciding on 2x6 walls or 2x4 walls with a layer of rigid foam. R-value is not a deterrent for either wall system which will meet or exceed any code requirements if you fill the cavities with high performance batts or sprayed insulation. Most of the heat loss through your walls will occur through the windows anyway.
2x6 framing is the simplest and cheapest option: frame the wall, stuff it with fiberglass, and you're done. Here are the downsides:
1. It's not very green. The additional wood is wasted from a structural standpoint.
2. Some thermal bridging will occur since the studs pass through the wall system, but wood doesn't conduct a great amount of heat. This is only a significant problem with steel studs.
3. Additional air sealing is recommended. In addition to a moisture barrier, you should either install a well sealed air barrier (which may be the same as the moisture barrier) or use spray foam insulation which can be expensive. In our area, some builders are spraying a thin layer of foam to seal the stud cavities and then installing a layer of batt insulation which is less expensive and allows for easier wiring and future rewiring.
I have always preferred the 2x4 framing option with an exterior layer of rigid foam. This system naturally blocks thermal bridging and is an efficient use of wood. You may need a separate moisture barrier depending on the foam you use (the Dow SIS claims to be a moisture barrier), but a layer of rigid foam with taped joints is an effective air barrier. There are still some downsides:
1. Depending on whom you talk to, rigid foam is not a particularly "green" product. Unlike glass fiber batts, it's made from petroleum products.
2. The siding will stick out awkwardly from the foundation unless you inset the framing from the face of the foundation, or you extend the rigid foam down the outside of the foundation which can become a conduit for insects.
3. Some extra framing is required around openings to provide nailing for windows and doors.
4. Depending on the foam you use, furring strips must be installed to attach your siding (I couldn't find any information from Dow SIS on this topic). If the strips are installed vertically, they can create a rain screen cavity which will extend the life of the siding and the exterior finish.
To sum it all up, I'd have to say I preferred 2x4 walls with exterior foam before spray foam became widely available. Now, I'd think twice about 2x6 walls with a combination of spray and batt insulation. On my home plan website, www.originalhome.com, you can select either 2x4 or 2x6 wall studs for your plans, and every customer chooses 2x6s.
Good luck with your project,
David Moore, AIA
Original Home Plans
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PS. You can find additional reading on the subject at New Home-Lumber Quality.