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Community and Q&A

Thermal bridging solutions when closed-cell foam is used?

Judy5 | Posted in Green Building Techniques on

What is the best way to handle the thermal bridging of the wood framing of my house — recognizing that some closed cell foam is specified for the wall cavities? We are constructing a house in climate zone 5 with 2×6 studs using the “hybrid wall assembly” as defined by Joe Lstiburek in his article “Hybrid Attics and Hybrid Walls.” I have seen no comments about managing thermal bridging using this type of wall system.

The specifics of our walls, from the inside out, are: drywall; R-11 unfaced batt insulation; 2″ closed cell foam insulation; 1/2″ plywood sheathing; WRB/Air Barrier (such as Henry Blueskin); vinyl siding.

We will have closed cell foam inside the wall cavities which will NOT allow the sheathing to dry inward. As a result, we cannot heed the advise (from “Can Exterior Foam Insulation Cause Mold and Moisture Problems?” and other articles) to use an adequate amount of external rigid foam insulation (approx. a minimum of 30% of the wall’s R value for zone 5) to keep the sheathing warm, because houses with excellent external insulation also must allow the sheathing to dry to the inside of the house.

Is there a product (presumably with high permeance) that could be used on the exterior that breaks the thermal bridging for both the studs and rim joists? If not, should we apply strips of foam to the interior sides of just the studs to minimize whatever thermal bridging we can?

Or should we just live with the thermal bridging — because too little exterior foam is worse than no exterior foam, and anything else we might do may just cause unintended consequences elsewhere?

Framing likely starts next week. As I have read in many questions, “I probably should have asked months ago …”

Your thoughts, comments and recommendations are greatly appreciated.

Thanks! ~ Judy

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  1. GBA Editor
    Martin Holladay | | #1

    You can install a continuous layer of mineral wool insulation on the exterior side of your wall sheathing if you want. Here are links to articles that describe this approach:

    "Installing Mineral Wool Insulation Over Exterior Wall Sheathing"

    "Installing Roxul Mineral Wool on Exterior Walls"

    "Wrapping an Older House with Rock Wool Insulation"

    Mineral wool is far more vapor-permeable than rigid foam, so a continuous layer of exterior mineral wool will still allow wall sheathing to dry outward.

    If framing starts next week, it's a little late to be choosing your wall insulation details. You may want to read this article: "Plan Ahead For Insulation."

    By the way, the type of insulation method you have chosen is usually called "flash-and-batt." In my article on flash-and-batt, I wrote, "The flash-and-batt approach doesn’t address thermal bridging through studs or rafters. While the nominal R-value of a flash-and-batt 2x6 wall with 2 inches of spray foam is R-25, the actual whole-wall R-value (after taking thermal bridging into account) is only R-17. Because of the thermal bridging problem, many insulation experts say that the flash-and-batt approach is a waste of good foam. In most cases, builders will get a bigger bang for their buck by installing a continuous layer of exterior rigid foam than by installing a thin layer of spray foam between the studs."

    For more information on this topic, see "How to Design a Wall."

  2. Judy5 | | #2

    Thank you! I will get reading!

  3. Expert Member
    Dana Dorsett | | #3

    At 2" most closed cell foam is still north of 0.5 perms, a class-II vapor retarder that WILL still allow drying toward the interior. You don't have any vapor retarders on the exterior, so it can also dry quite readily to the exterior behind the inherently back-ventilated vinyl siding. In zone 5 the vinyl siding would meet code from a drying perspective with just a Class-III vapor retarder on the interior (standard latex paint).

    Why low-density R11s (which are not very air retardent) rather than R13s or R15s?

    What is the purpose of the 2" of closed cell foam?

    You would get about the same thermal performance (and better drying) as your R11 fiberglass + R12 closed cell foam using R23 rock wool or dense-packed fiberglass, or a full cavity fill of 0.7lb density open cell foam.

    At the same wall thickness as a 2x6 studwall, dropping back to 2x4 studs and installing 1.5" wide strips of 2" thick Huber ZIP-R on the interior side of the stud edges would add R9.6 of thermal break to the framing fraction and provide a half-inch OSB nailer for hanging the wallboard.

    This is similar to using 1.5" foam board Bonfigilioni strips as the thermal break:

    Taking the Bonfiglioni approach with 1x furring the total wall thickness would be 1/4" deeper than with a standard 2x4, so you'd be looking at 5.75" cavities, not 5.5" as with a standard milled 2x6. But if you're filling it with open cell foam 1/4" of depth isn't a problem, whereas with batts designed for 5.5" cavities it can leave voids. At 5.75" half-pound open cell foam would run about R21, 0.7lbs goods would be R23. A full cavity fill of half-pound foam would cost about as much as the 2" of closed cell foam you were specifying. You would be slightly lower-R at center cavity than your R11 + 2" closed cell foam cavity fill, but with 1.5" foam the framing fraction's R would be roughly doubled.

    With the faster drying toward the exterior due to the back ventilated vinyl siding, in zone 5 you would not need any sort of interior side vapor retarder.

    Also note, 5.75" of half pound open cell foam uses less polymer than 2" of closed cell foam- even 5.75" of 0.7lb foam uses slightly less polymer than 2" of closed cell foam. But open cell foam is blown with water rather than HFC245fa (a powerful greenhouse gas, ~1000x CO2), so even the full fill of 0.7lb foam would be a much greener solution than HFC blown closed cell foam.

  4. Expert Member
    Dana Dorsett | | #4

    At the high cost of rigid rock wool, if it's not possible to drop back to 2x4 framing at this point you can probably still thermally break the framing with interior Bonfiglioni strips with 1-1.5" foam and go with 7.75-8.25" of open cell foam, delivering higher thermal performance, higher resilience, and still cost less.

  5. JC72 | | #5

    @Jon R.

    Wouldn't a 1/2 air gap behind the cladding rectify that concern?

  6. Jon_R | | #6

    I haven't seen any data that indicates that 1.5" of external unfaced EPS (perhaps 2 perms) isn't beneficial and less risky than foam strips on the interior side (ie, cold sheathing). It's easy to find evidence that too many external side perms (such as common WRBs with or without mineral wool insulation) can cause inward moisture drive problems (but perhaps only with moisture retentive sidings).

  7. Jon_R | | #7

    Yes, but it leaves the question of why pay more for high perm mineral wool if somewhat lower external perms (but > 1 (such as EPS) to allow outward drying) work better.

    See below for some significant benefits to even 1" of foam and "The most important factor to observe is that adding exterior insulation benefits the wall insulation more significantly than optimizing the vapor permeance of the WRB...". Seems reasonable to lump foam/mineral wool perms with WRB perms.

  8. Judy5 | | #8

    Thank you, everyone, for your comments. Happily, it is not too late to change to 2x4 framing. I have been reading the links, and rethinking our strategy. The windows (for 2x6 studs) are already on site, but we can manage whatever the final decision is. I hear the message that open cell foam is better than closed cell foam. The BSC study indicates that one inch of external EPS may not create a "foam sandwich" because it has enough permeance to still dry to the exterior. I like the Bonfiglioli approach as an interior option. So we appear to have some options still available. Thank you all for your collective guidance.

  9. GBA Editor
    Martin Holladay | | #9

    Note that if you go with the flash-and-batt approach, you'll have to use closed-cell spray foam, not open-cell spray foam. For a variety of reasons, open-cell spray foam isn't suitable for use with the flash-and-batt approach.

    Of course, if you use a continuous layer of exterior rigid foam, you won't need any spray foam whatsoever.

  10. Expert Member
    Dana Dorsett | | #10

    At $2 per square foot for just the foam that 2" closed cell flash'n'batt really isn't "worth it". Spend the foam budget for the insulating sheathing instead, where you get the full R performance out of it.

    In Zone 5 as little as 1.5" of foil faced polyiso (~ R10 ) is sufficient dew point control at the sheathing for up to R23 (rock wool batts- much cheaper than closed cell foam or rigid rock wool) in the cavities.

    With foil facers you get an additional ~R1 out of it if there is nothing but air between the vinyl siding and shiny foil facer, which gives it even more margin.

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