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Community and Q&A

Foam sandwich?

Joanie21 | Posted in Energy Efficiency and Durability on

MY insulation specialist and contractor say there’s nothing to worry about moisture-wise with their 1″foil faced poly-iso on the exterior of plywood sheathing and 4″ closed cell foam in between 6″ studded wall.  No house wrap.
Windows are top of the line Alpen R9 quad paned and I’m all heat pumps and ERV.  I just upgraded my siding to the engineered Thermory wood T&G siding and am considering using their proprietary snap on rain screen system.

HELP!  I fear that the plywood gets one drop of moisture from, say screws racking (river wind)  and leaving a gap in rigid foam, wets the plywood and the moisture has nowhere to go inside or out to dry.  Did I mention Parapets,  Yeah the worst but I wanted solar & the town wants ’em hidden, sigh. That flat Roof is getting 1/2″ iso and a ton of closed cell foam underside. Would have used more rigid exterior roof but then I would have needed DensDeck and I had serious height restrictions (neighbors river views)

I have a feeling my contractor just doesn’t want the challenge of innie windows if I load up on the exterior rigid foam and change interior insulation to something more vapor permeable.  They say they are giving me the most economical solution with the most Rvalue and that if the siding does it’s job the plywood will NEVER get wet. WITH NO HOUSE WRAP other than this foam sandwich.  Am I crazy or will this actually work?

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  1. Expert Member
    Dana Dorsett | | #1

    A few points about this stack up:

    1> Closed cell foam between studs is a waste. For the parallel-path math on why, see this:

    Save the high R/inch foam budget for the exterior, where it's performance isn't being robbed blind. A full fill of 5.5" of 0.7lb. open cell foam and NO exterior polyiso will come in at about the same performance.

    2> At 4" closed cell foam is still not a vapor barrier- typically 0.2-0.3 perms- there is still a drying path toward the interior.

    3> At 4" closed cell foam is structural- it may not matter if your sheathing rots away- you have 4" thick plastic glued to the studs keepint it from racking.

    4> Foil faced polyiso is not automatically a weather resistant barrier (WRB), but can be detailed to make it perform that function. The window flashing has to extend to the exterior side of the foam if the foam is the WRB.

    There has to be a better, more resilient, cheaper higher performance solution here. Got a ZIP code?

    1. Joanie21 | | #2

      Thanks for the quick reply. We are kinda in the 11th hour. ZIP code 10706, Hastings-on-Hudson, NY just north of the City with epic views...and wind of the mighty Hudson. Another item I forgot was that Solar will be on self-ballasted racks tilted only 10% south. That's why I couldn't go higher with rigid on the exterior flat roof. If I did, the weight of solar racking system (no boots) would demand DensDeck and then we're above the parapet & neighbor goes nuclear. I'm kinda committed to the closed cell foam on underside of that roof I've got 12" to play with. NYS code is R49

      1. Expert Member
        Dana Dorsett | | #5

        Hastings is in US climate zone 4A, and no exterior exterior foam for dew point control or interior side vapor retarders (other than standard interior latex paint on wallboard) are required on 2x6 walls. Filling the stud bays with dense-packed cellulose or fiberglass, or 0.5-1lb open cell foam would meet or beat the IRC's code minimum R20.

        To hit R49-equivalent performance and meet code on U-factor basis only takes R36-R37 of continuous insulation. That could be 6-7" of 2lb roofing polyiso above the structural roof deck. Reclaimed/used roofing polyiso is dirt-cheap, and available from multiple vendors in your area:

        If you're going to split the R between above deck and underside insulation it takes at least R15, which could be 3" of roofing polyiso above the roof deck, and R29 (8" of half-pound open cell foam or 7.25" of rock wool batting- there are other options that work) snugged up to the underside of the roof deck. It really can't take another 2-2.5" above the roof deck? A half inch of iso above the roof deck buys you next to nothing on performance.

        It it milled 2x12 rafters, which are really 11.25" deep, or is it a true 12". With milled 2x12s a complete cavity fill of 0.7lb open cell foam would run about R45, at which point 3/4" of polyiso above the roof deck gets you to R50. At 11.25" the vapor permeance of 0.7lb foam is less than 5 perms which should be protective enough, but code would demand a Class-II interior side vapor retarder. That could be a sheet of 2-mil nylon (Certinteed MemBrain), or "vapor barrier" latex primer on the ceiling gypsum.

        (Not that it matters, my brother lived in Hastings for a dozen years or so, and both his kids did their high school carreers there back in the '90s- GREAT schools!)

  2. GBA Editor
    Martin Holladay | | #3

    It sounds like you are in Climate Zone 5. If you want to install rigid foam on the exterior side of a 2x6 wall in your climate zone, you need the rigid foam to have a minimum R-value of R-7.5, so 1 inch of polyiso isn't quite thick enough. For more information, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    I agree with Dana that the use of closed-cell spray foam between studs is a waste. More information here: "Installing Closed-Cell Spray Foam Between Studs is a Waste."

    To avoid the "foam sandwich" problem, you can either use a "fluffy" insulation between the studs (cellulose, fiberglass, or mineral wool), or you can switch from closed-cell spray foam to open-cell spray foam (which is vapor-permeable). For more information, see "How to Design a Wall."

  3. Expert Member
    Michael Maines | | #4

    Siding always leaks, especially in windy locations. The house should be water-tight before any cladding goes on. Rigid foam alone does not make a great WRB, in my opinion--it's safer to use a separate product. It doesn't need to be fancy; plain old felt paper or Tyvek will do, and allow for mechanically lapped seams.

  4. Jon_R | | #6

    Consider cellulose, unfaced 1.5" EPS foam and WRB for the walls - reasonable drying in both directions.

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