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Advice on open cell in exterior walls, zone 5

css1813 | Posted in General Questions on

We are going to be constructing a new home later this year and the topic of insulation has come up. I have done quite a bit of research and reading, including many articles here, and unfortunately still not sure which way to go. We will have 2×6 studded exterior walls (2 ft on center studs), with 1/2 inch sheating, Tyvek home wrap, and vinyl siding on the exterior face, and drywall on the interior face of the walls. I know this is pretty standard, and I think we’ve narrowed down the project to use either entire open cell foam in the 2×6 walls, or go with a hybrid option which is 1.5 inches of closed cell with 4 inches of cellulose. Originally I liked the idea of the open cell foam more as I have read a lot of good feedback about it, however one of the local insulation companies I have talked to says they do NOT install open cell in our area and prefer the hybrid option. They are telling me that open cell absorbs too much moisture and they prefer the closed cell option to act as a moisture barrier. I have read that it is good to let the walls “breathe”, and have also seen reports of the closed cell and cellulose hybrid option causing condensation inside the walls. I posed this question to this company and they claim that with 1.5 inches of closed cell foam that this problem does not happen in our climate.

So…. my question is, for our climate in zone 5, is open cell a good option or should it be avoided, considering the building materials being used here? I am hoping for some good advice which will help us make a better decision. Thank you very much!!

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  1. GBA Editor
    Martin Holladay | | #1

    Either option will work without any moisture worries. Building codes in your climate zone require the use of an interior vapor retarder (for example, vapor retarder paint), which will help your wall avoid moisture problems.

    For more information on the flash-and-batt (or, in your case, the flash-and-fill) option, see this article: Flash-and-Batt Insulation. This type of wall dries to the interior.

    Walls don't have to "breathe." They just have to be well detailed.

  2. css1813 | | #2

    Thank you for the helpful feedback. I figured the drywall itself would act as the interior vapor retarder/barrier but it sounds like it will need some extra help from the paint.

    I read your article "Installing Closed-Cell Spray Foam Between Studs is a Waste" and was convinced that open cell is definitely the way to go. In this case by going with flash-and-fill, the cost is higher, but not significantly higher because they are only proposing 1.5" of closed cell. In your opinion, if we place the flash-and-fill option (closed cell) next to the full open cell option, which is better for our climate overall?

    One final question... if we go with the flash-and-fill option, do we still need vapor retarder paint?

    Thank you for this excellent resource, your articles are extremely helpful.


  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "I figured the drywall itself would act as the interior vapor retarder/barrier but it sounds like it will need some extra help from the paint."

    Your latest explanation is better than what you originally figured, but you're still a little misinformed. Drywall is vapor-permeable. It is never a vapor retarder. It is, however, a good air barrier -- and air leakage is much more worrisome than vapor diffusion. In the case of drywall painted with vapor-retarder paint, the drywall is a vapor-permeable substrate. The layer that limits vapor diffusion is the paint.

    For more information on this issue, see All About Vapor Diffusion.

    To answer your second question -- "Which is better, flash-and-fill or open-cell spray foam?" -- I would answer two ways. From an environmental perspective, the open-cell spray foam option is better, because most brands of closed-cell spray foam use a blowing agent that has a high global warming potential.

    From a thermal performance perspective, the flash-and-fill approach is slightly better. The spaces between the studs will have an R-value of about R-20.35 with open-cell spray foam, or R-23.80 with flash-and-fill.

    But as I noted in the article you mentioned (Installing Closed-Cell Spray Foam Between Studs is a Waste), thermal bridging through the studs undermines both of these options. The next time you build a house, install a continuous layer of exterior insulation (mineral wool or rigid foam).

  4. css1813 | | #4

    Thank you very much for the explanation. That helps me quite a bit in dealing with these local insulation companies, as they all have a slightly different way of doing things. Having this education will help us try and make the best decision here.

    I was also told that by using closed cell, it would form as the vapor barrier that we need to have whereas open cell can't do that even with 5.5 inches of it (for a 2x6 wall).

    I appreciate your help and again would like to comment about the great value of this website.

  5. GBA Editor
    Martin Holladay | | #5

    Flash-and-batt (or flash-and-fill) walls don't need an interior vapor retarder because the closed-cell spray foam is a vapor retarder. The elements of the wall assembly on the interior side of the spray foam are designed to dry inward. The elements of the wall assembly on the exterior side of the spray foam are designed to dry outward.

    Building codes require, and building scientists recommend, that a framed wall insulated with open-cell spray foam in a cold climate have an interior vapor retarder. Vapor retarder paint on the drywall will meet this requirement.

  6. css1813 | | #6

    Thanks Martin, that is a great explanation and makes perfect sense. Again I appreciate all of your help!

  7. Expert Member
    Dana Dorsett | | #7

    In zone 5 a wall with vinyl siding has NO requirement for an interior side vapor retarder, since vinyl siding meets the code definition of "vented cladding" in the exceptions list:

    A "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) or half perm paint (aka "vapor barrier latex")) wouldn't hurt, and would limit the alleged moisture accumulation problem associated in the open cell foam stated by the installer.

    For the cost of the 1.5" closed cell foam between the studs you could probably install 1.5-2" of polyiso on the exterior, which would be more than sufficient for dew point control at the sheathing level for R20 open cell foam or cellulose in the cavities, without needing an interior side vapor retarder. (Again, see TABLE R702.7.1 in the link.) Exterior side foam would deliver a substantial upgrade in thermal performance as well, not quite doubling the "whole wall R", but more than 1.5x, a reducing the heat flow through walls by more than a third.

    R20 is the IRC 2015 code MINIMUM, not the optimum. It's the crummiest performance that is code-legal to build. If for similar money to the flash 'n' fill you can go open cell in the cavities and R7.5 or greater on the outside, the latter is by far the better way to go.

  8. css1813 | | #8

    Thank you Dana, I can definitely see your point about the exterior foam as you and Martin have both mentioned it. I will have to look in to this option to see what is available here. My only concern might be the extra distance that the foam adds to the wall thickness, and how they will handle the windows and doors to accommodate. And, what is the better material to act as rainshield on the outside of the foam itself (I've seen some discussion about that in this forum). This is all probably a question for our builder which I will ask to see what options we have with it.

    Thanks again for this valuable help and resource.

  9. Chaubenee | | #9

    My advice is to put foam board on the outside and dense pack your walls with cellulose. Air seal the sheathing with tape and housewrap then attach your exterior foam.

  10. Expert Member
    Dana Dorsett | | #10

    "My only concern might be the extra distance that the foam adds to the wall thickness, and how they will handle the windows and doors to accommodate."

    A 2x4/R15 wall with 2" of exterior polyiso is exactly the same wall thickness as a 2x6/R20 wall, but higher performance. There are still some detailing differences around windows & doors though.

    There are lots of tips on this site about the detailing for even fatter foam walls, starting here:

  11. Reid Baldwin | | #11


    I think most people on this forum would agree that a wall with exterior foam is better than either of the options you are considering, even if it means 2x4 studs instead of 2x6 studs to maintain the same wall thickness. It sounds like you would need to push your builder out of his comfort zone to make that happen. Whether or not that is worth it depends on how badly you want a better than code minimum house and on how flexible your builder is. Some builders are open to doing something new. Others won't change unless you are adamant and they don't have enough other customers. Don't assume that an insulation contractor would be the one installing the exterior foam. On my house, the framers did it.

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