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The vapor permeance of exterior foam and other questions?

joenorm | Posted in General Questions on

I am building in climate 4C Marine.

I’d like to apply a layer of exterior insulation on the outside of the WRB.

For a variety of reasons including ease of install, cost, minimal complications, etc. I want to use the minimum thickness, just enough to ge the benefit of the thermal break.

For a 2×6 wall I found an article that says I need R-3.75 for this in my climate.

This can be achieved with 3/4″ Foil faced Polyisocyanurate which is labeled as R-4.4.  Climate 4C is pretty mild temperature wise, do we see the cold weather performance issues with Polyisocyanurate here? Or is that a concern only in much colder zones? It also feels counterintuitive to be putting foil on the exterior of house when we go to the trouble of applying a breathable membrane as a WRB.

What about EPS foam without foil? It is more permeable(correct?) and costs less. Is this a more suitable material for exterior insulation? In my climate what would be the minimum thickness I could use on the exterior walls? Even if it had to be 1 inch thick, this would not affect construction details too much.

thanks for your thoughts here

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  1. Peter Yost | | #1

    Hi Joe -

    I think the article you are referencing is this one from GBA:

    So, yes you have the R-value of the exterior insulation for your climate correct.

    In terms of the vapor permeance of insulation of a certain thickness or the per unit thickness vapor permeability of insulation I am attaching a good resource.


  2. joenorm | | #2

    What are the thoughts here about foil faced polyiso as an exterior wrap?

    Foil is essentially impermeable. Seems like a risky setup to wrap a building with something that does not allow outward drying.

  3. Expert Member
    Akos | | #3

    If you want to go with the foil faced insulation, it would be best to get a crinkly house wrap to allow a bit of drying. Without the seams taped, the foam will still allow a bit of drying.

    There are lot of foam products out there with higher perm rating, having extra drying capacity never hurts, worth the extra legwork/cost.

    I'm a fan of fiber faced roofing polyiso as it is the cheap and permeable. The drawback is the WRB has to go over the foam, which is harder to detail if you want to keep it a standard build (ie flanged windows mounted onto the sheathing). With the thin insulation you need, I doubt the material cost is worth the extra labor.

    1. joenorm | | #5

      Akos, So what would be the product you recommend? You're right in that I want to keep it standard(Plywood, windows, housewrap, then foam)

      I am trying to strike a balance of gaining the benefit of the thermal break without causing too many construction detail headaches. Hence trying to keep the foam as thin as possible not to cause any problems.


      1. Expert Member
        Akos | | #9

        If available, your best bet for thin foam is:

        Long term R5.6 per inch.

        Otherwise go with unfaced EPS. Crinkly housewrap is not a big upcharge and might be worth it no matter which insulation you choose.

  4. Jon_R | | #4

    While more is better, there is no minimum thickness - with the right design, you can use any amount, including none. Yes, breathable unfaced EPS is a better choice than non-perforated foil or film covered.

    See here for a general guide. Or just use 1" of unfaced EPS and don't worry about it.

    1. joenorm | | #6

      Jon R,

      GBA articles say there is a minimum thickness of R-3.75 in my zone.

      1. Jon_R | | #7

        That is incorrect. Some things the article says are simply wrong and some are unclear to the point where they are widely misinterpreted. Some of it was corrected here.

        You can certainly use R1 of exterior foam - it can be code compliant and not suffer from any moisture problems.

  5. GBA Editor
    Brian Pontolilo | | #8


    The minimum thicknesses serve to prevent condensation on the sheathing. Akos and Jon have made two good suggestions, the drainable housewrap will promote drying and will diminish performance insignificantly and EPS will offer more drying potential that foil faced foam. However, there is one important detail that you have not mentioned, unless I missed it. Does your home have an vapor barrier (poly sheeting) on the interior. If it does, the stakes are higher. If it doesn't, your walls can dry inward and adding exterior foam is less risky.

    1. joenorm | | #11


      The home is under construction so does not have a vapor barrier and I do not plan to add one.


  6. Expert Member
    Dana Dorsett | | #10

    >"This can be achieved with 3/4″ Foil faced Polyisocyanurate which is labeled as R-4.4. Climate 4C is pretty mild temperature wise, do we see the cold weather performance issues with Polyisocyanurate here? "

    The average wintertime temps in most of zone 4C are high enough that no temperature derating is needed. Even looking at Karagiozis' worst case sample you'll note that the thermal conductivity at the labeled 75F/24C average temp through the foam layer is the same as the conductivity at ~ 54F/12C through the layer, and that it's less conductive between those two temperatures:

    Dow chemical's worst-case sample in Figure 2 of this document shows it meeting labeled spec at +48F average temp through the sample.

    BSC's average of samples of roofing foam from different vendors show only a ~5% derating from the labeled-R when the mean temp through the foam is 35F instead of the tested 75F performance:

    Mean wintertime outdoor temps in zone 4C tend to run near 45F over the 12 coolest weeks, give or take a few degrees, with long shoulder seasons where the temps are warm enough that the foam will outperform it's labeled R. eg:

  7. joenorm | | #12

    In the article it says 1" EPS is R-3.6 per inch. Is this close enough to the suggested R-3.75 to be considered safe even at the "older" standards?

    I read the articles, interesting stuff.


    1. Jon_R | | #13

      R-3.85 is a more accurate value for the most common 1" EPS sheets. Some others are > R-4. All will have an even higher R value at the temperatures that matter for moisture control. Not an important difference, but you can check the specs for the specific insulation you intend to buy. Higher densities are also more durable.

      I recommend that you spend much more time on air sealing and testing than this issue.

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