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Is it OK to sandwich exterior wall sheathing between external foam board and interior flash coat closed cell spray foam?

I am building a new energy efficient home. The builder wants to have the exterior wall pre-fabricated and for structural stability wants to have the 2 x 6 walls sheathed with 1/2" OSB. The exterior wall would then get 1' XPS or polyiso, a Tyvek rain wrap, and then cement board vertical siding.

The interior will receive a 2" flash coat of closed-cell spray foam and the cavity finished with batt insulation.

I don't see a path for moisture to get to the OSB or plywood, but was interested in opinions on this procedure. Thanks for you input

Asked by ed kemp
Posted Apr 18, 2012 2:49 PM ET
Edited Apr 23, 2012 9:40 AM ET


7 Answers

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yes i like foam plus the fiber glass insulationas apose to all foam

Posted Apr 18, 2012 7:27 PM ET


I don't see a path for moisture to get to the osb or plywood

But it may get there just the same. Especially with vertical siding, which is more difficult to detail correctly around window and door openings. If your home is single-story construction with deep eaves and in a local climate which does not include wind-driven rain you'll probably be OK - otherwise better to allow the OSB to dry in one direction or the other.

Answered by James Morgan
Posted Apr 19, 2012 6:41 AM ET


NO!!! You never want to sandwich wood sheathing between XPS and closed cell. I would install blown cellulose or blown FG in the wall cavities, and air tigth drywall. When you install foam board on the outside, you need the wall to dry to the inside. Depending where you live, you may need more than 1" of outsulation.

Answered by Armando Cobo
Posted Apr 19, 2012 7:51 AM ET
Edited Apr 19, 2012 7:53 AM ET.


Armando says: "Depending where you live, you may need more than 1" of outsulation."

I believe Armando is referring to limiting condensation from the flow of solar heat from outside and humidity from inside. The idea is to keep the sheathing warm enough (climate dependent) and dry.

I'm in an area that gets considerable wind and rain at times. So I'd be very reluctant to sandwich the sheathing w/o drying capacity to, at least, the inside via the drying capability of cellulose.

By the way, XPS has a much, much larger carbon footprint than does polyiso rigid foam board.

Lots of info on each of these topics here, especially in the enegy nerd blog, for background. (I'm not a builder, fyi.)

Answered by 5C8rvfuWev
Posted Apr 19, 2012 9:38 AM ET


It's best not to sandwich OSB or plywood between two layers of vapor-impermeable foam. One solution: substitute open-cell spray foam for closed-cell spray foam. However, since you can't use open-cell spray foam for a flash-and-batt job, your spray foam contractor would have to fill the entire stud bays with spray foam if you go this route.

To be sure that your proposed layer of exterior rigid foam is thick enough, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Answered by Martin Holladay
Posted Apr 23, 2012 9:43 AM ET
Edited Apr 23, 2012 9:48 AM ET.


Martin...I have foam board on the outside of my osb and 2x6 wall...I've seen where it is recommended to use foam in the rim joists. Does this apply to this same scenario? Our foam board comes down to below the same level as the rim joist to the concrete foundation. Can I still go forward with foam in the rim joist? Will there be any problems? Also, I have a walkout basement. On the walkout side is it better to still use foam or foam board in the rim joist or can fiberglass be used there? I'm assuming the main concern is air sealing in smaller spaces? I'm using dense fill propink in the remaining wall assembly upstairs and foam foard on the concrete basement walls. Aron

Answered by Aron Robinson
Posted Feb 11, 2013 7:10 AM ET


If you have an adequately thick layer of rigid foam on the exterior side of your rim joist, you don't have to add additional thermal insulation on the interior. If you are worried about air leakage, you can always address cracks with caulk.

However, if you want to take a belt-and-suspenders approach, you can install spray foam on the interior of your rim joist. Open-cell foam might be better than closed-cell foam in your case, since you have an exterior layer which is fairly impermeable to water vapor.

If you insist on using closed-cell spray foam, I advise (1) doing it on a day when the rim joist has a very low humidity level, and (2) using only a thin layer -- perhaps 1 or 2 inches.

Answered by Martin Holladay
Posted Feb 11, 2013 8:25 AM ET
Edited Feb 11, 2013 8:26 AM ET.

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