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Zip-R sheathing and spray foam?

I am looking to build my house in Birmingham, AL, Zone 3. I plan on using Zip-R 1 1/2" (possible 1" due to cost) and use 2x6 advance framing minus the single top plate. The exterior will have brick veneer.

My question is about using spray foam in conjunction with the Zip R polyiso. Is it chemically compatible? Thought on just filling up 24" cavities with open cell vs 2" CC and R15 batts?

I am a little worried about the CC not allowing drying to the inside, however, I'm not sure, since it will be bonding with the polyiso.

The windows will be U.27 SHGC 0.17. Unfortunately, the house will face due South. There are a lot of windows, on the front, but plan good overhangs to help out. Some of the windows will fit directly in the stud bays as they will be 22" wide.

Any other suggestions?

Tripp

Asked by Paul Pankey III
Posted Mon, 02/24/2014 - 22:52
Edited Tue, 02/25/2014 - 08:15

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2 Answers

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1.
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Paul,
Zip-R sheathing has a permeance that is below 1 perm; for all intents and purposes, it is a vapor barrier.

You are correct that the polyiso layer of Zip-R sheathing faces inward. Since any building with Zip-R sheathing can't be expected to dry outward, adding closed-cell or open-cell spray foam on the interior side of the polyiso layer doesn't change anything -- at least not with respect to the drying direction.

So you can go ahead and use spray foam between the studs if you want to.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/25/2014 - 08:41

2.
Helpful? 0

The open cell approach is fine- there is hardly any point to going higher-R with 2" cc foam + R15 batts. Despite the higher center cavity R it adds at best R3 to the whole-wall R.,

That would almost certainly be a more expensive R3 than the R3 boost you get going from 1.0" ZIP-R to 1.5" ZIP-R.

As in most of these analyses, applying the high-R insulation budget into the sheathing-R is likely to be a better bang/buck than putting high-R insulation between thermally bridging studs & plates that robs it of performance.

Answered by Dana Dorsett
Posted Tue, 02/25/2014 - 16:44

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