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Polyiso / spray foam / insulation / paint?

easyrider470 | Posted in Energy Efficiency and Durability on

I hate to continue asking what seems to be a different wording of the same question but bare with me. My house is 2×6 exterior walls, 1.5 inch of polyiso rigid fiberglass faced foam outside the sheathing and Tyvek. We are in the 47460 zip.

I would like to know the best way to achieve the tightest wall possible but still allow my sheathing to dry accordingly. 1-2 inches of closed cell in the stud void is something Dana said would be OK, so I am leaning towards that but want to be 100% sure it’s OK.

Second question is….have I severely limited my options for types of paint on the drywall since I used the polyiso outside the house? I am wondering what type of interior paint is safe to allow my walls to dry correctly.

So re-cap…..can I flash the sheathing in the stud voids with 2 inches of closed cell foam for a complete air seal….OR am I better off with just doing the wet sprayed cellulose and hoping that the taped seams of the polyiso have done a good air-sealing job?

I would prefer to use the wet sprayed cellulose on top of the foam instead of batts because of the labor time and cost. I can have cellulose sprayed in for .80 cents a ft compared to batts at .32 cents for material then I need to install them myself. My time is worth more than .50 cents a ft so the cellulose wins.

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  1. GBA Editor
    Martin Holladay | | #1

    The continuous layer of rigid foam insulation that you plan to install on the exterior side of your sheathing is a good idea. Make sure that you comply with the recommendations in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You can use any kind of paint you want on the interior of your house. However, you should stay away from vinyl wallpaper.

    Installing spray foam between your studs isn't the best use of your money, because the thermal bridging through your studs largely negates any benefit from the foam. I would use dense-packed cellulose in the stud bays if I were you.

  2. easyrider470 | | #2

    Thanks Martin. The house is already built and we are about to begin insulating this week. I can save quite a bit of money by not using the spray foam in the stud cavities. I have 1.5 inches of thew poly iso which is well within the minimum R value for my climate zone.
    Thank you for clarifying the paint issue as well, I was worried.
    Last, do you think that the polyiso itself is air tight enough to not make the house leaky through the Cellulose?

  3. GBA Editor
    Martin Holladay | | #3

    There are lots of ways to create an air barrier. You can tape the polyiso; you can tape the plywood or OSB sheathing; you can use lots of caulk and canned spray foam; or you can follow the Airtight Drywall Approach.

    Regardless of which way you go, it's useful to conduct at least one blower-door test to verify your air sealing efforts.

    Many builders are gravitating toward the taped sheathing approach -- either using Zip sheathing or ordinary plywood or OSB. For more information, see Airtight Wall and Roof Sheathing.

  4. wjrobinson | | #4

    Foam foam foam foam... some foam good so add lots of foam.... nope. not so good,

    One air barrier good... ten air barriers... not good.

    Building Science has many example walls explained.

  5. easyrider470 | | #5


    Any chance you can give me an idea of the thermal bridging loss I would have if I chose to do the 2" closed cell foam in the stud voids of my exterior walls?
    16" o.c. 2x6 walls, 1/2 inch sheating with tyvek, 1.5" polyiso rigid foam taped seams

  6. Expert Member
    Dana Dorsett | | #6

    The thermal bridging is not much affected by what you put between the studs as long as whatever it is has has a higher R-value than wood.

    Leaving an partially filled cavity increases the thermal bridging due to the shorter path through the wood. Filling up that last bit with bubble gum or cat hair is about as good as filling it with cellulose or 2lb foam from a thermal bridging point of view, since bubble gum & cat hair still have R values equal to or higher than the thermally bridging wood. Filling up that last inch of cavity increases the conduction path through the wood, which adds about R1-R1.2 to the framing fraction, which is typically 25% of the total wall area. Since the wood is the most conductive path, anything you can do to maximize that path length reduces the rate of heat moving through the thermal bridge, but once you're at full stud depth you're as good as it's going to get. Increasing the R-value of the cavity fill that covers 75% of the wall area helps the total heat loss numbers, but doesn't affect the amount of heat moving through the framing.

    At a 25% framing fraction and R1.2/inch wood, when the R-value of the cavity fill is 3x that of wood (or about R3.6/inch, or R20 in 2x6 framing) the wood is conducting as much heat as the cavity-fill. Even if you filled the cavity with magic foamed mouse-milk that gave the cavity an R value of R1,000,000, the wood is still conducting the same amount of heat. Even though R-million is 50,000 times greater than R20, the wall's performance is only about 2x better, not 50,000x better. That is because the wood is still conducting as much heat as it did with the R20 cavity fill, which was half the total.

    And that's why you get better bang/buck applying the foam budget to continuous insulation over the exterior, since there it adds it's R value to both the framing fraction as well as the cavity fill fraction. Instead of having only ~ R6 at the framing fraction, you have R6 + Rfoam at the framing fraction. Even 1.5" of exterior EPS cuts the thermal bridging losses of a 2x6 wall in half, while padding out the center-cavity R a bit. And since the thermal bridging framing is responsible for half the heat loss, that's a better than 25% improvement in wall performance.

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