Can I use Zip System R sheathing if I keep the relative humidity low?
I’m quite drawn to the Zip System R sheathing for reasons of installation, cost, and ability to install cedar shingles without horizontal strapping every 5″. Unfortunately, based on the many blogs and discussions from this site, it seems that even the thickest Zip R product (1″ polyiso) is insufficient exterior foam sheathing for climate zone 6a, given our preferred wall system (2X6, dense packed cellulose).
However, I did some calculations using the formula in Ted Cushman’s 2006 article, ‘Robust Walls’, and it seems like I can pull it off if I keep the interior relative humidity around 28% during the heating season. Does this seem like an unreasonable or ill advised approach?
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Replies
Mike,
You will not be the only homeowner who occupies the building over the next 70 or 80 years. The next homeowner may not maintain the interior RH according to your principles, and the result may be moisture accumulation and rot.
You need to abide by the recommendations for the minimum R-value of your exterior wall foam: Calculating the Minimum Thickness of Rigid Foam Sheathing.
The Big IF
"If you plan to install exterior rigid foam on the walls of your house, how thick should the foam be?"
Every time I see this question come up ... I wonder how many people actually notice the "IF" ???
I think it should be noted that Exterior Foam is not REQUIRED ....
Many high performance homes include NO exterior foam at all.
Yabbut, Martin, in 70-80 years global warming will have moved the exterior R requirements DOWN! :-(
Penetrating 1" foam with 10,000 shingle nails cuts seriously into any enhanced thermal advantage you might be gaining there. It's just WAY too many highly thermally conductive fasteners. From a thermal performance point of view (but still prone to issues) you'd be better off going with 2x4 framing and using 2" iso or EPS under the gypsum on the interior, since the number of penetrating fasteners would be an order of magnitude lower. Better yet use 2x4 framing and 2.5" of exterior iso held in place with a staggered seam OSB nailer on which to hang your shingles (or 3" nailbase iso), which has the advantage of thermally breaking the band joists & subfloors.