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Foam sandwich in Canada

_Stephen_ | Posted in Energy Efficiency and Durability on

Hi Martin,

I have a question about upgrading insulation in our home we’re having built in Southern Ontario.

The “normal wall” the builder uses is 1″ of XPS rigid insulation on outside, installed over OSB, on top of a 2*6″ wall on filled with R18 fiberglass bat. Naturally, the whole thing is sealed on both sides, with tape on the XPS and poly sheeting on the inside.

I want to replace the fiberglass with closed cell spray foam, a full 5″, to really up the insulation factor, and not screw up anything the builder wants to do, as this isn’t really a custom home, but they’re willing to accommodate my desire for energy efficiency within limits.

However, it seems like either wall construction can’t dry anywhere… Won’t this rot??

What’s the right way to deal with this?



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  1. GBA Editor
    Martin Holladay | | #1

    The short answer is no -- the wall probably won't rot. One inch of XPS has a vapor permeance of 1.1 perm, which allows outward drying (albeit at a very slow rate).

    The more nuanced answer is that any wall can rot. The biggest moisture threat to any wall is rain, so keeping your wall from rotting depends on (a) good flashing details, especially at window rough openings, and (b) including a vented rainscreen gap between the siding and the water-resistive barrier (WRB).

    You never asked an important question: "Is this a good wall?" The answer to that unasked question is no -- this isn't a very good wall. The wall could be improved by switching from XPS to EPS (because EPS is manufactured with a blowing agent that is more environmentally benign than the blowing agents used to manufacture XPS), by making the rigid foam layer thicker (to keep the OSB sheathing above the dew point in winter), and by replacing the interior polyethylene with MemBrain (a so-called "smart" vapor retarder with variable vapor permeance).

    There are lots of articles on GBA that address these issues. Here are links to a few:

    How to Design a Wall

    Installing Closed-Cell Spray Foam Between Studs is a Waste

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Smart Vapor Retarders for Walls and Roofs

    Do I Need a Vapor Retarder?

    All About Rainscreens

    Choosing Rigid Foam

  2. _Stephen_ | | #2

    Hi Martin!

    You've given me a lot of think about. and read about! I especially enjoyed reading the 'Closed cell spray foam between studs is a waste" article... That's a great article. I'll definitely be switching to filling the studs with open cell spray foam!

    How difficult is it to increase the amount of rigid foam on the exterior for my builder? And if I can't get them to install the apparently required 4" of rigid foam, is 2" better than 1"?

    Thanks so much! My membership here is totally going to be worthwhile during this process.


  3. GBA Editor
    Martin Holladay | | #3

    Q. "How difficult is it to increase the amount of rigid foam on the exterior for my builder?"

    A. Not difficult, as long as your builder is willing to spend some time on the GBA site. The issue probably has more to do with cost than difficulty -- since some builders reflexively charge more for new techniques than necessary, simply out of fear of the unknown.

    Q. "If I can't get them to install the apparently required 4 inches of rigid foam, is 2 inches better than 1 inch?"

    A. You're probably in Climate Zone 6. (Here is a link to a Climate Zone map including Canada.) In your climate zone, the rigid foam on the exterior side of a 2x6 wall needs a minimum R-value of R-11.25. (See the table in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.) You can achieve that with about 3 inches of EPS or about 2.5 inches of polyiso -- so you don't need 4 inches of rigid foam.

    But you've got the right idea -- in general, thicker rigid foam is safer than thinner rigid foam. Here's the wrinkle: Two inches of XPS would not necessarily be better than 1 inch, because it slows down the drying rate to the exterior without being thick enough to protect the sheathing from moisture accumulation during the winter.

  4. Expert Member

    My own recent experience is that there is quite a large difference in both cost and complexity moving from a thin layer, (say up to 1 1/2") of foam to a 4" layer. The switch in fasteners from being able to use a nail gun to washer-head screws when securing the strapping, the diligence necessary to locate the underlying studs, flashing and trimming openings - all add up to making the job a quite different animal.

    That isn't to say it it isn't the right thing to do in many circumstances, but I was quite surprised when I went to deal with thicker foam how much the tasks differed.

  5. GBA Editor
    Martin Holladay | | #5

    Good points. I plead guilty to minimizing the hassles and costs of installing thicker rigid foam. Your answer is better than mine.

  6. _Stephen_ | | #6

    So, if I get the builder to install EPS to the maximum thickness that the builder will go (Somewhere between 1 and 3 inches), switch to open cell spray foam, and install a MemBrain for the vapor retarder

    Does this then qualify as a "good wall".

  7. GBA Editor
    Martin Holladay | | #7

    Once again, I direct your attention to Calculating the Minimum Thickness of Rigid Foam Sheathing. If you want to install exterior EPS, the minimum thickness is 3 inches.

    While one inch of rigid foam (as long as it isn't foil-faced) may not be disastrous, it's not desirable. You want to follow the minimum R-value guidelines in my article.

    And 2 inches of EPS is definitely worse than 1 inch of EPS. Aim for 3 inches.

  8. Expert Member
    Dana Dorsett | | #8

    Using 2" of an R5/inch graphite loaded EPS (not XPS) can still work OK in a US zone 6 type of climate, but its marginal. It's performance increases enough at outdoor temperatures cold enough to matter to provide dew point control at the sheathing. With 5.5" open cell foam in the cavities there is little or no air-transported moisture, and MemBrain or painting the wallboard "vapor barrier latex" would be cheap insurance when the exterior R is marginal.

    XPS loses performance over time as it's (climate damaging HFC) blowing agents leak out, and would eventually be not quite enough R at 2", even though it would probably be enough for the first 5-10 years. What starts out as R10 (R11-ish during the depths of winter) eventually drops to R8.5-ish (R9.5-ish during the depths of winter). EPS (graphite loaded or other) has a stable performance over decades.

    At 1" of exterior foam the MemBrain might still save you, but 2-mil nylon doesn't have a 50+ year track record in this application. With foam that thin you might be better off with 4-mil polyethylene on the interior, despite how much that impedes the overall drying capacity, or half-perm latex. Somebody well versed in the proper use of WUFI would be able to simulate the different options.

    With 1" of unfaced EPS on the exterior, installing 1" of closed cell foam on the interior (leaving a ~4.5" gap for the fiberglass) and compressing the R18s into the cavity would also work. The vapor retardency of the closed cell foam would be about 1-perm (give or take), while the 1" EPS would be 2+ perms. The lower vapor retardency of the closed cell foam keeps interior moisture drives from loading up the sheathing with moisture in winter, and the R10-R11 stackup of 1" EPS + sheathing + 1" closed cell foam is more than sufficient dew point control for 4.5" of fiber insulation, keeping the fiber insulation from loading up on wintertime moisture.

  9. Expert Member


    I made exactly the same assumptions as you going into the last job I did and only learned of all the additional work once I got into it.

    One of FHB's Houses of the Year was built with thick exterior foam and exposed stick framing as the finished interior. After having missed my fair share of studs screwing the foam, I have a hard time imagining a less forgiving way of building.

  10. _Stephen_ | | #10


    We may be screwed then. Ontario building code requires R-5 of continuous insulation, which is almost always installed as 1" of rigid foam insulation.

    I doubt I can get them to do either 3" of rigid foam or a double wall, which would also work to satisfy the requirement. Or at the very least my builder is likely to want far too much money to do it, making it too expensive.


  11. Expert Member
    MALCOLM TAYLOR | | #11

    The use of 1" foam on the exterior is now ubiquitous on new construction in Ontario and Quebec. It will be interesting to see the results in a few years time. That's a pretty big sample group to test the dangers of inadequate levels of exterior insulation.

  12. Jon_R | | #12
  13. Robert Opaluch | | #13

    Malcolm points out an important problem, attaching rigid insulation fasteners or vertical rain screen strapping to narrow studs, through thicker rigid insulation plus sheathing. Not attaching bolts solidly to the studs might not be noticed, with bolts holding initially to the sheathing and edge of studs, but wind, gravity or deterioration might take a toll some day.

    One solution to more reliably and easily bolt rain screen strapping or exterior insulation board fasteners:
    2x4 horizontal strapping applied to the exterior of a sheathed stud wall, preferably 27.5”o.c. Insulation board can be placed between the strapping. More insulation board could be added with vertical rainscreen strapping attached (through the additional insulation board) to the horizontal 2x4 strapping, instead of all the way to the studs.

    1. Ease of installing insulation board fasteners or vertical rain screen strapping. Fasteners can be installed more easily and reliably to the 3.5” wide horizontal strapping compared to the 1.5” wide studs;
    2. Relative ease of installing 1.5" thick, 2' wide insulation board products in the 24" gaps between the 2x4 horizontal strapping (e.g., Roxul ComfortBoard R-6, R-6 EPS, R-7.5 GPS graphite-loaded EPS, or R-9 polyiso);
    3. Structural support underneath 2' wide rigid insulation board installed between the 2x4 horizontal strapping (ComfortBoard is heavier and needs more support, and any insulation board products would stay put while installing between the 2x4 ledges);
    4. Ease of adding additional layer(s) of EPS, GPS or polyiso insulation board, since insulation fasteners can be shorter, and installed easily and reliably to hit 3.5" wide horizontal 2x4 strapping instead of 1.5" vertical stud faces. The 2x4 horizontal strapping surface is 2” wider and 2" closer to the exterior than the studs.
    5. Horizontal strapping could be aligned below window bucks to provide some support for heavier windows; or just above the windows; or near the base of the wall for attaching any plywood bottom below thicker insulation board layers. 2x4 horizontal strapping could provide additional support for any window unit or window bucks above it. Near the bottom of the wall, horizontal strapping could provide a nailing surface for any plywood used to protect along the bottom of insulation board.
    6. Vertical rainscreen strapping or insulation board fasteners no longer must be aligned with vertical studs, which would reduce the thermal bridging in the wall. Metal fasteners are terrible transmitters of thermal energy. (Even if vertical strapping fasteners are aligned with the studs, they do not need to penetrate all the way through the horizontal 2x4 strapping into the studs, reducing thermal bridging.)

    Disadvantages include:
    1. More material and labor costs for 2x4 horizontal strapping. (Although costs partially offset by using some shorter fasteners, and by faster installation of rain screen vertical strapping or rigid insulation fasteners);
    2. A thermal bridge is created through the 1.5” thick horizontal 2x4 strapping (compared to continuous 1.5” insulation board). But the problem is mostly limited to the intersection of 3.5” wide strapping and 1.5” vertical faces of wall studs, a small percentage of the wall (similar to the Mooney wall).
    3. Technically the horizontal strapping makes the 1.5” layer of insulation not a continuous insulation layer, thereby some green building requirements or incentives are not met without additional insulation board.
    4. Horizontal 2x4's @27.5” o.c. may be insufficient for more than 1.5” ComfortBoard or for structural support for strapping to support heavy siding?
    5. Possibly some cases where vertical strapping fasteners require more than 1.5” depth into solid wood framing

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