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Community and Q&A

Can spray foam be used as a WRB?

Howard Klein | Posted in Plans Review on

We are building a ~6000 square foot, 2 story above ground house with a 3000 sqft basement. The house is located in Montreal. The weather here can go from -25C (-13F) in the winter to very hot and humid 32C (90F) in the summer.

The proposed wall assembly from exterior to interior is as follows

5″ of stone
1″ air space
2″ closed cell spray foam
Plywood sheathing
2×6 , 16″ OC filled with blown in cellulose dense pack or blown in fiberglass
Membrain by certainteed (Everyone freaks out at the thought of not having poly, so this was the compromise)
5/8″ Drywall

1) Besides the cost of spray foam , is there any reason NOT to use it a WRB? Every new house going up in Montreal is using closed cell 1″ on the outside of the house to air seal the structure.
2) Where can I find details how to flash the windows to the spray foam, as the windows will likely be installed before the foam is applied?
3) Most people on this site seem to prefer foam board instead of spray foam for the exterior – is this because of cost or some other reason?


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  1. GBA Editor
    Martin Holladay | | #1

    The main reason that builders prefer exterior rigid foam to exterior spray foam is that exterior rigid foam is usually cheaper. Another advantage: the scheduling is under the control of the builder rather than a subcontractor.

    As long as the spray foam is dense enough, and as long as the spray foam is conscientiously applied, there isn't any reason why it can't be your WRB. After all, spray foam is used as roofing.

    The difficulties come when you try to integrate window flashing, door flashing, and penetration flashing to your WRB. In theory, roofing foam is "self-flashing." However, there are so many possibilities for job-site error on the part of the spray foam installer that I would never depend on the spray foam installed on a wall to be self-flashing.

    Most jobs with exterior spray foam have some type of vertical furring strips -- often installed on stand-offs -- that provide something solid to attach the siding to. In theory, you should be able to flash your window rough openings with some type of flashing that directs any water to the exterior of your rigid foam or your siding layer. You'll probably need sheet metal flashings for your rough window sills if you go this route, though. The details are somewhat daunting, but a good roofer or siding installer with a brain should be able to figure them out.

  2. Howard Klein | | #2

    Thanks for this.
    Besides the details you mentioned - is there anything else "wrong" with the wall assembly?

  3. Expert Member
    Malcolm Taylor | | #3

    Howard, a couple of other potential problems with using spray foam on the exterior come to mind.

    The first is that when spay foam is used to completely fill wall cavities it is flattened after it dries with a scraper. Otherwise it is very irregular and not uniformly thick. Your exterior will be this way and it may be very difficult to provide a consistent drainage plane and air space behind your rock facing.

    Anchoring the rock to the wood framing is also potentiality difficult. Presumably you are using some sort of ties similar to those for brick, which are secured to the framing behind. These would have to penetrate the foam.

    There may be easy solutions to both concerns, I just bring them up as things to think about.

    Good luck with your house.

  4. Expert Member
    Dana Dorsett | | #4

    Closed cell foam sprayed in 2" lifts is pretty smooth- FAR smoother than open cell foam which has many times the total expansion. If has been uses successfully for decades as the WRB on more exotic shapes like dome-homes, and as long as it's protected from UV light degradation, hangs in there pretty much forever.

    SFAIK NOBODY over-fills framed cavities with 2lb foam and trims it flush as is common with half-pound foam- the stuff is just too damned hard!

    On roofs 3lb density is typically used, for better walking-robustness, but for the WRB in a masonry cavity wall, 2lb good are just fine.

    In the climate described 2" is probably sufficient to be able to skip the interior vapor retarder, especially with cellulose in a 2x6 cavity, but there is no down-side to using MemBrain or it's competitors. Poly would be a disaster in waiting, since 2" of 2lb foam is fairly vapor retardent, which could be a potential moisture trap.

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