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Continuous exterior insulation and interior spray foam

Islander6 | Posted in General Questions on

Is there a problem in terms of vapour control if i use 3-4 thick polyscio taped joints for air/vapour barrier/wrb and spray foam closed cell 2 inches on the inside. Vapour barrier on the outside and one on the inside

There is enough insulation on the exterior to keep the dew point outside of the framing, so does it really matter. The spray foam has the benefit of acting as a second air barrier which is why i like it

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  1. Expert Member
    Akos | | #1

    There are WAY cheaper ways to get a second a 2nd air barrier (ie tape the sheathing).

    The 2" of ccSPF also does next to nothing for the R value of the wall assembly. You are probably looking at an R2 bump at the most.

    1. Islander6 | | #3

      Confused as to how 2" is useless and only and r2 bump if its rater R6 per inch

      1. Expert Member
        Akos | | #5

        The spray foam in between wood studs which are R1/inch. Because of the thermal briding from the wood, the effective R value of SPF+stud is much less. You can read about it here:

        The SPF is also not that much more than batts (R3.5 to R4.2 / inch), taking into account the stud thermal conductivity, you are only increasing your assembly R value by a small number.

  2. Expert Member
    BILL WICHERS | | #2

    I would use batts in the walls here to keep costs down. I like mineral wool myself, but there are many other options and any will work. Detail your drywall as an air barrier, or add a smart vapor retarder like membrain and detail that as your air barrier. Spray foam between studs in walls is really wasted for insulating value, and there are far cheaper ways to do the air sealing.

    Spray foam is an EXCELLENT product in certain applications where it's unique properties make it the best way to go. Walls are generally not one of these places.


    1. Islander6 | | #4

      Where do you recommend spray foam and im confused as to why its no good in walls? Would it be useful for underneath frost protected slab, as i am doing this also and inteded to use rigid foam but i can see how spray foam would work also

      1. Expert Member
        BILL WICHERS | | #6

        Spray foam is the most costly insulation out there. Spray foam is the best way to do an unvented roof, and can be useful in basements and crawl spaces against very rough walls (like stone foundations). These are places where the unique properties of spray foam really make it the best option.

        In walls, the thermal bridging of the studs kills the performance of the spray foam. You end up with a relatively small performance gain as a percentage here, and it’s bit enough to justify the increase in cost to go with spray foam. You can get MUCH better overall wall performance by spending the extra money on exterior rigid foam, for example.

        I’d use rigid foam under a slab too. Best would be reclaimed EPS or XPS, next best would be new materials of one of those types. EPS is the greener option of two, but if you’re using reclaimed materials just use whichever is available. Reclaimed materials really shine in any application where they’ll be hidden from view and can be sealed using sloppy methods (like canned foam). The reason is that reclaimed rigid foam usually has some damage to the sheets, and that damage makes the install look a little off, and can make taping the seams more difficult. Under a slab neither of those things really matters though, so you might as well save some money and use reclaimed foam.


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