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Should I use two layers of 1 inch or one layer of 2 inch rigid foam board on the exterior?

Should I use two layers of 1 inch or one layer of 2 inch closed cell rigid foam board on the exterior? Locally we have available foil-faced on both sides. My understanding is to have only the exterior side foil faced. If using two layers of 1 inch, then should the board against the stud be unfaced?

I've read the articles on rigid board insulation on the exterior and concerns of condensation. I live in a warmer part of zone 5B at 3,400 ft. elevation.

The existing 2x4 walls built in 1987 are insulated with a 7/8" open cell board in the inside with fiberglass batt insulation, the exterior wrapped in kraft paper with a redwood lap siding. The plans are to retain the 1/2" drywall and 7/8" open cell foam board. Working from the outsiide the fiberglass will be removed and replaced with closed cell spray in foam, then OSB, Tyvek house wrap, with 2" closed cell rigid foam board with metal siding.

Asked by Brad Dorken
Posted Tue, 11/20/2012 - 12:23
Edited Tue, 11/20/2012 - 12:41

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3 Answers

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Brad,
If you read the article on minimum foam thickness (Calculating the Minimum Thickness of Rigid Foam Sheathing), then you know that the minimum R-value for a 2x4 wall in your climate zone is R-5, so 1-inch of polyiso will work. Two inches is better.

Two layers of rigid foam with staggered seams is better than a single layer of the same total thickness, because the staggered seams do a better job of air sealing and reducing thermal bridging at the foam seams.

Don't worry about whether the rigid foam has one layer of aluminum foil, or 2 layers, or 4 layers. One layer of aluminum foil has a very low permeance, and so do 4 layers. One layer or 4 -- it makes no difference.

The extra layers of aluminum foil help reduce the rate at which the blowing agents inside the polyiso escape to the atmosphere, which is a good thing.

Whenever some asks me whether it is a good idea to encapsulate OSB sheathing with low-permeance foam on both sides, however, I advise them to make one side of the OSB sheathing more permeable. If you want to install exterior rigid foam -- and I think you should -- then I don't advise using closed-cell spray foam in your stud cavities. Choose open-cell spray foam instead -- it's more permeable to water vapor.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 11/20/2012 - 12:50
Edited Tue, 11/20/2012 - 13:55.

2.
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Leave the fiberglass, or replace it with open cell foam (or blown cellulose). If you put in closed cell foam and foil-faced rigid on the exterior the structural sheathing will be in a moisture-trap, independent of whether the adjacent rigid layer has a facer or not.

The additional performance of closed cell foam in a stud cavity is remarkably low, due to the thermal bridging of the studs. You can put only about 3" of cc foam in a 2x4 stud, since it isn't easily trimmed, and the 3" thermal bridge of the framing is an R3 shunt for 20-25% of the total area that is otherwise R18+. That reduces the average whole-wall R of the cc foam case to about R10 & change, whereas a full 3.5" open-cell case trimmed flush to the studs is about R9.5. How much are you willing to spend for another R0.5? The open cell foam ( or fiberglass or cellulose) still allows the sheathing to dry toward the interior, whereas the closed cell foam does not (or rather severely restricts that drying.)

The average temperature of the sheathing at center cavity is related to the ratio of the R value of the stackup exterior to the sheathing and the R-value on the interior side. In your stackup the R-value of that 7/8" goods on the interior counts.

As long as the total R on the exterior is sufficient to keep the January mean temp at the sheathing above the dew point of the conditioned space air , you do not need a strong interior side vapor retarder. (The dew point should be presumed to be no lower than ~37F in zone 5, which would correspond to a 30%RH/70F conditioned space. In a tight house even 40F+ dew points are common unless controlled by the ventilation rate to keep it lower.) The IRC specs an R5 minimum insulating sheathing for standard 2x4 construction, R7.5 for standard 2x6 construction.

Since you have insulating foam on the interior in addition to the cavity fill, your center cavity R inside the sheathing is probably approaching R20, which would make it more comparable to the 2x6 center-cavity R, which means you'd need more than 1" of exterior foam, independent of facer or foam type, but 2" (any type) WOULD meet code.

There is no advantage to using unfaced goods at the studs when double-layering it, and it's easier to get reliable air seals on foil facers with FSK tape than any methodfor air-sealing unfaced foam board.

The higher the exterior R, the more margin you have. Foil faced EPS is cheap per inch, and would meet the bare minimum at about R8. But using foil faced polyiso would run R12 @ 2". And even though EPS gains performance with falling temps and iso loses some, the crossover point at any given thickness is well into negative double-digits F. Derated for your average January temp iso would would run ~ R11/inch, whereas uprated for that temp EPS is only~ R9. And it's the average that makes the difference in how much moisture accumulates in OSB.

Answered by Dana Dorsett
Posted Tue, 11/20/2012 - 13:16

3.
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Thanks so much for your answers that help me and the contractor clarify this issue I will use a open cell spray foam for the center cavity and 2" of polyiso on the exterior.

Answered by Brad Dorken
Posted Wed, 11/21/2012 - 02:36

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