Will a 12″ double 2×4 wall with fiberboard sheathing perform well in a Cold Climate?
I am building a house using double 2×4″ wall construction. I would like to avoid using rigid foam insulation at the exterior if possible. I was contemplating using a 3/4″ Temple-Inland structural fiberboard, Tyvek, and a 3/8″ rainscreen. The fiber board has a perm rating of 15 or so. With a vapor barrier paint at interior and dense pack cellulose 12″ thick this wall would dry to interior and exterior. I live in Madison WI. According to the 2007 IRC this works in zone 6. The only thermal bridging that is designed into it is a 3/4″ plywood top plate and at windows and doors. I want to avoid rigid insulation as much as possible because of the global warming potential during manufaturing (XPS being the worse, EPS and Polysio slightly better). Any insight on the merits of this type of construction? Thanks!
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I'm not familiar with that particular sheathing material, but given the properties you state for it you should be just fine without exterior foam. The whole point of that is to offset the thermal bridging of wood stud framing, and your double wall design does just that nicely. Adding the rainscreen element to the wall just makes the overall package that much better by helping the siding dry more easily and by keeping water that does get behind the siding out of the cellulose in the cavity.
I think your proposed wall is fine. Am curious though where you got the data for the rigid foam.
Sam,
Thats a great wall for a heating climate. A classic example of diffusion open to the exterior. It's very good as it is but I'd like to suggest a couple of details to consider...
1, Consider adding a something more reliable than than vapor barrier paint. Properly done, your wall can last centuries vs decades. By placing a vapor barrier + air barrier well behind the drywall, it wont matter what future generations do do the interior drywall layer in terms of additional coats of paint, perforations etc. Both the air and vapor barrier will be safely behind the finish layer.
There are various ways of making the service cavity: Place OSB on the backside of the interior studwall and tape it. Tilt it up onto gaskets (resource conservation technology).
2, Do make that envelop air tight. It's not out of reach with a double stud wall to easily beat the Passive House air sealing requirements of 0.6ACH 50. Just draw the airbarrier as a continuous line around the envelope and plan the material or method for each transition or field.
3, The fact that you've got a rainscreen is great. There are various schools of thought on how deep the cavity should be. Deeper is better when you consider that a larger gap allows more airflow for venting and evaporation to buffer the temp swings on the exterior sheeting . And in the case of fibre board exterior sheeting, the dense packed insulation (be it cellulose of fibreglass) will put outward stress on the fibreboard and cause it to buldge between the bays. We've seen that happen here in the Pacific Northwest often. Your choice of 3/4" fibreboard will lessen the bulging, but I really do suggest a 3/4" rainscreen gap and caution against a 3/8" gap.
Sam,
You have a very good wall design. With an R-40+, heat loss through the wall will be minimal. I like 16" oc 2 x 4 studs for a double wall, lumber is reasonable these days and this will help with the bowing Albert described with dense pack. Airtightness is the other essential detail when building high performance homes. You can go with ADA, or a continuous, warm side polyethylene air barrier would be fine for WI. With fiberboard sheathing this wall will dry to the cold side, just the way it should.