Skinnier double stud assembly- does it affect moisture performance?
I am designing a home in Zone 5 where the client initially wanted 2″ Zip-R outside of 2×6 walls. That would have given us about R30 with 7.5″ thickness for structure plus sheathing. For various reasons we have been considering going to a double stud wall. We could use 2 layers of 2×4’s placed fairly close together to give us an 8″ thick wall with a similar R30. I’ve read through many of the articles and discussions about double stud walls, but the articles with data on moisture performance refer to projects in zone 6 and use an R45, 12″ thick wall. I’ve also seen a few details with 10″ walls and R40. Would the lower R-value or smaller about of moisture-buffering cellulose make an 8″ thick double stud wall assembly more susceptible to moisture problems, or would the greater movement of heat through the wall actually help dry the assembly?
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I think the potential for problems increases as the depth increases. Take an extreme case... a 10 ft. thick wall. In that case, there is a larger area of the wall cavity that is below the dew point.
Just one thing I see often overlooked with double stud wall assemblies... the gap between the studs means you need fire blocking every 10 ft. of horizontal wall space. Just a friendly reminder. :-)
Hallie,
As Patrick says: the decrease in temperature of the sheathing in a thick wall isn't offset by the moisture buffering of more cellulose. In either case though, with good air-sealing and the ability to dry to the exterior enhanced by a rain-screen gap, there don't appear to be problems with double walls of any width.
Much of the point in using double stud walls is the ability to gain more depth for insulation by widening the gap, without having to increase the amount of framing. If the wall is only going to be 8", perhaps using 2"x8"s makes more sense?
The other problem with a very small gap is how it gets insulated. You can offset the studs, but what happens between the bottom and top plates, and at openings?