How to calculate insulated sheathing thickness?
I am designing a new house in Rhode Island. We are using 2 x 6 studs with 4 in. closed cell spray foam insulation in the walls, with 5/8 in Zip sheathing (taped) over the studs, 1 1/2 in EPS styrofoam over the sheathing and an outer layer of 5/8 in. Zip sheathing (taped) and cedar shingles as the exterior. Second layer of Zip sheathing is fastened with long screws into the frame.
The interior is 5/8 in. blueboard and plaster, no vapor barrier. The roof is 7 in. closed cell spray foam between the rafters with 3 in. of styrofoam and the same two layers of Zip sheathing (taped). Cedar shingles with Cedar Breather. Interior ceilings are blueboard and plaster, no vapor barrier.
I am concerned about condensation trapped in the outer insulating sheathing in winter
whether driven from the interior or, more likely, from the exterior.
Is this safe with 1 1/2 in of EPS styrofoam in the walls and 3 in. in the roof, given that this is Zone 5 coastal?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Read this yet? It may help. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
This seems like a very complicated and resource-intensive wall assembly. Have you thought about double stud walls with cellulose?
Jim,
Q. "I am concerned about condensation trapped in the outer insulating sheathing in winter, whether driven from the interior or, more likely, from the exterior."
A. You won't get condensation on your exterior OSB, but the OSB will be at a high moisture content during the winter, simply because it is cold. For more information on the cold OSB question, see How Risky Is Cold OSB Wall Sheathing?
I agree with Dan that your proposed wall assembly sounds expensive and complicated. You a proposing a site-built SIP on the exterior of a spray-foam-insulated stud wall. I'm a little worried about your interior layer of OSB, because it will be sandwiched between two layers of relatively impermeable foam. If you like to use foam, whether rigid foam or closed-cell spray foam, it's usually best to put it just one place -- either on the exterior side of the wall sheathing, or the interior side of the wall sheathing, but not both.
I dont understand the outer layer of ZIP. Is it to have something for the cedar shingles to nail too? I would think horizontal strapping would work, although not a good rain screen design...
But yes, as Dan said, a double stud wall would probably be a good option. Give you solid exterior nail, simpler to build and detail, and will most likely be a lot cheaper both materials and labor. A double stud wall with ZIP is my choice in zone 6 and it is the cheapest way to hit r35+ for me.
Of just go with SIPs.........