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Exterior Rigid Foam Under Sheathing?

Joshua Greisen | Posted in Green Building Techniques on

In my local area there is a green building consultant that recommends a CI panel which is basically EPS foam glued to OSB. The way this is installed would be the OSB to the exterior. Almost every house I see with exterior rigid foam in my online wonderings is covered with blue and pink XPS, the OSB sheathing is nailed directly to the studs and is completely covered with the rigid foam. This leads me to the following questions:

– Is anyone familiar with these kind of CI panels?
– Is there any benefit to putting the sheathing on the exterior side of the rigid foam as seen in these products?
– Why does adding rigid foam on the exterior of house sheathing seem more prevalent than sheathing to the exterior?

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Replies

  1. James Morgan | | #1

    Sheathing is normally applied directly to the studs to provide structural resistance to diagonal racking of the wall framing. A layer of ESP interposed between the studs and the OSB breaks that direct mechanical connection, so the arrangement your consultant describes would only pass inspection if a) the panel is specially engineered to provide such bracing - look up insulated structural sheathing - or b) if the framer includes alternative bracing provision such as let-in diagonals or proprietary steel bracing elements.

  2. John Clark | | #2

    Google ZIP-R ZIP SYSTEM from Huber. It's slick stuff.

    Benefits: Ease of construction. Less steps required = lower chance of error. Easier to built a rain screen, no messing around with house wrap (at least with ZIP-R), only have to tape seams once.

    The one downside is that sheets which have thicker pieces of foam (ex R12) will require additional structural bracing for the framing. So if you wanted to do a really high R-value wall it might be easier to just throw up OSB first and then attach the foam.

    Here is a link of a GBA contributor who is in the process of finishing his house in Georgia (CZ3) and he used ZIP-R.

    https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/topping-out

  3. GBA Editor
    Martin Holladay | | #3

    Joshua,
    Here is a link to a GBA article that explains everything you need to know about the type of wall panel you are talking about: Nailbase Panels for Walls.

    1. Mykcuz2000 | | #15

      Is there any code compliance issue with installing 1"xps to the studs, followed by 1/2"ply sheathing and tyvek? Wouldn't this be a cheaper alternative to huber zip r?

  4. Joshua Greisen | | #4

    Martin,

    Thank you for the link, that is the article I was hoping to find. I wouldn't have thought of the keywords "Nailbase Panels" though. I kept searching under CI panel.

  5. Joe Suhrada | | #5

    Put the foam on the outside. Not BETWEEN the studs and OSB. You won't regret it.

  6. Joshua Greisen | | #6

    Joe,

    Sticking with the foam to the exterior of the sheathing is the direction we are heading. Now we are digging into the details on the exterior EPS panels. Trying to figure out the most cost effective product in our area that meets our requirements. Whether the product should be faced for the walls /foundation and whether a single thick layer versus 2 overlapping thinner layers is better. Any advice in these areas?

  7. Joe Suhrada | | #7

    I used 2.5 inch single layer white EPS but two overlapped layers are certainly a little better. I taped the seams with 3M all Weather tape and since I used reclaimed foam, any little gouges or voids, I added some Great Stuff I need a can. It is remarkable how much warmer the OSB is when it has the foam outside.

  8. Steve Knapp CZ 3A Georgia | | #8

    Joshua,

    Also consider using reclaimed foam on your project. It is much less expensive than new material and is more environmentally friendly. One source is Insulation Depot or, possibly, your local Craigslist.

  9. Joe Suhrada | | #9

    I second Steve Knapp. I found a guy in Oneonta NY on the Craigslist there. He has a business model that buys foam from demolition contractors tearing down the last remnants of industry in the northeast. Vast industrial warehouses, refrigerated warehouses, massive factories, old shopping malls and the such all over New York State and Pennsylvania are being put to the wrecking ball each day, and the demo contractors are keen to recycle and scrap, obviously. He buys the stuff they get off the roofs generally. I bought very nice two and a half inch EPS for my walls and also some high density three inch XPS that I used under my concrete attached porch slabs of which much of my house has on the perimeter. Nonetheless it is call Hickory Street Rentals. He even delivered it to me in Saratoga County and ultimately I saved half or more. I figure I rate the EPS at R eleven or so.

  10. John Clark | | #10

    @Joe

    Why would you not want to have the foam layer on the interior side of the sheathing? It really makes no difference.

    Exterior foam is a compromise. It's more work, requires more material, more difficult to hang a rainscreen, flashing details can be more complex.

    The only advantage of exterior foam is cost and the fact that you can push the R-value a little higher if you need it (most of the time you don't).

  11. Expert Member
    Malcolm Taylor | | #11

    John,
    There are two advantages I can think of. With foam on the outside the sheathing can supply the necessary shear resistance to the walls, and keeps the sheathing warmer. How much the second matters depends on climate.

  12. John Clark | | #12

    @Malcom.

    Agree. Additional shear bracing may be required with thicker levels of foam. That and cost are the primary issues (R6+).

    IMO the warmth of the sheathing is only a problem to be solved when using exterior foam in general.

    If the sheathing is on the exterior of the foam then drying isn't a concern because interior vapor will never reach the sheathing and the sheathing will always dry to the exterior (especially with any rainscreen). IMO that's a win-win.

  13. Expert Member
    Malcolm Taylor | | #13

    John,
    I just brought up climate because of our (perhaps unique) situation here. In the PNW exterior sheathing sustains a bit of damage over time simply because of the high outside humidity levels over a large part of the year. Exterior foam would alleviate that.

  14. Hugh Weisman | | #14

    Thinking of using EPS 1-1/2" thick under exposed floor framing with foam insulation...then MDO plywood as finish on top of (or rather beneath) the EPS. Can plywood be nailed or screwed ok to TJI framing above without any problems, or is some type of nailer needed? Also, how would you secure the EPS to the bottom of theTJI's before adding the plywood below? Cap nails? adhesive?

  15. Expert Member
    Zephyr7 | | #16

    I'm guessing this is part of a basement project, so you're planning to install a layer of EPS under the TJI joists, then plywood under that, secured with screws into the lower chord of the TJI joists? I don't see a problem here, unless you need a fire-rated ceiling assembly which would probably require a layer of drywall.

    I would use regular construction screws for this. I'd tack the EPS up with a few light-duty screws first, then put the plywood over the EPS using enough regular construction screws. I would stagger the sheets a little so the edges of the EPS and plywood don't line up.

    Note that you don't want to penetrate into the web -- the OSB center -- of the TJI joist. If you pick screws that will only go 1" or so into the lower chord you'll be OK. Most of the restrictions with TJIs have to do with cutting holes in the web, or cutting the chords (don't ever cut the chords). Nailing or screwing into the chords isn't usually a problem.

    Bill

  16. Hugh Weisman | | #17

    thanks....actually, we completed the work about a week ago...the contractor tacked up the EPS with some PL adhesive after installing 2 x 4's to support all edges of the 4'x8' MDO sheets...then power nailed the MDO sheets...came out good.

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