GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

What would you do?

up_state_ny | Posted in GBA Pro Help on

I’m building a house in climate zone 5.  I’ve got a little problem w/ the insulation.  I’ve got all the windows and electrical meter box furred out 1.5” for R-7.5 foam (2×6 walls), which is the code minimum and could lead to some moisture accumulation on the interior side of the sheeting.  I was thinking of furring the windows out another 0.75” and using 2.25” eps foam.  A local manufacturer produces eps foam to order.  My contractor thinks I’m crazy for increasing the foam.   So my question is, should go with the 1.5” or 2.25”.

The other little problem I have is shown in the picture.  The window adjacent to the sunroom is on the north west side on the house.  I have some concerns w/ the window so close to the roof edge.  Is there any benefit to placing the foam on the inside of the wall?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Jon_R | | #1

    > could lead to some moisture accumulation

    I'd say "almost certain to lead to some moisture accumulation". But probably not enough to be a problem.

    > which is the code minimum

    Code allows (in various ways) any amount of foam - including none.

    My opinion is that building with minimum listed R value external foam should not be done with low perm (eg, foil faced) foam. But is fine with > 1 perms (eg EPS) - because this allows some outward drying, further reducing risk. Better than code min is a good thing.

  2. Expert Member
    BILL WICHERS | | #2

    2” polyiso will get you around R13, and even derated for extreme cold, around R10. On my own house (also zone 5), I’m working on putting up 3” of polyiso (about R19), and that’s on 2x4 walls with R15 mineral wool.

    I’d use at least 2” of polyiso in your case. If you’re concerned with drying and don’t want to put enough rigid foam to be safe in that way, you might want to consider rigid mineral wool panels. The mineral wool panels allow a lot of airflow so there would be no problem with outward drying.


  3. GBA Editor
    Martin Holladay | | #3

    Your post is confusing. It sounds as if you are planning to install EPS. Most brands of EPS are rated at R-4 per inch or less.

    You describe 1.5 inch of EPS as R-7.5, but it isn't. It's probably R-6.

    You need to install a minimum of R-7.5 of rigid foam to be safe. For more information on this issue, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    Assuming you install rigid foam on the exterior with a minimum R-value of R-7.5, then it is untrue that the rigid foam "could lead to some moisture accumulation on the interior side of the sheathing." The R-7.5 rigid foam will keep your sheathing warm and dry.

    You could use 1.5 inch of polyiso or 2 inches of EPS, and you'll be fine. If you use foil-faced polyiso, don't worry -- this type of wall doesn't need to dry to the exterior.

    Your last question -- about "the window adjacent to the sunroom" -- is confusing, but I think you are worried about all of the water that will be coming off the roof above the window. If that is your worry, remember that you need kickout flashing at the bottom of the roof, where the step flashing begins.

    1. up_state_ny | | #4

      As shown in the picture you pasteed in your response, illustrates the window in question.

      To the right of that window, where the two roof lines meet, is the common wall between the house and garage.

      As shown in the photo the roofing and flashing are completed.

      My question is, can I put 2" of foam on the inside wall. The reason why I want to do this:
      1.) It seems like the foam will be discontinuous around the sunroom.
      2.) I feel that moisture will get behind the foam and start leaking into the window opening(if one did an exceptional job flashing around the window, this would reduce potential leakage, however I question the installer's attention to detail)
      3.) There's really no need to place the foam on the garage wall (this project has a finite budget)

      It just seems like a better use of material to place the foam on the inside wall for this location only. If I do follow through with this strategy, is there some unintended consequence that I'm not thinking of. I stated earlier I'll be using 2.25" of 25 psi xps foam.

      Thank you for your help.

      1. GBA Editor
        Martin Holladay | | #6

        Your original post referred to 2.25 inches of EPS. Your most recent comment referred to 2.25 of XPS. These aren't the same -- I'm not sure what type of foam you are talking about. For more information, see "Choosing Rigid Foam."

        It's possible to install rigid foam on the interior side of an insulated wall, if you want to do that. (Note that I don't recommend installing rigid foam on both sides of your wall.) For more information, see this article: "Walls With Interior Rigid Foam."

    2. Jon_R | | #5

      > is untrue that the rigid foam ... The R-7.5 rigid foam will keep your sheathing warm and dry.

      False. I've provided the calculations when this has come up before. There will be times where the sheathing is below the dew point and during these periods there is some moisture accumulation. It won't always be dry, although overall it will probably be dry enough.

      1. GBA Editor
        Martin Holladay | | #7

        You can provide the "calculations" as many times as you want, but your calculations won't change the facts. I stand by my statement that the minimum R-values recommended for exterior rigid foam will keep your sheathing warm and dry. If you want to refresh your recollection of my recommendations on this issue, here is the link to my article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

        No one is claiming that there won't be a few hours in January when the sheathing temperature is low enough to allow for some sorption. The issue that building scientists have looked at is the following: Does the sorption that occurs during those hours cause any problems? And the answer is no, it doesn't. This conclusion has been repeatedly verified by field studies, again and again, that include moisture meters inserted into sheathing. When exterior foam with the minimum R-value shown in the recommendations is installed, the moisture content of the sheathing stays low -- much lower, in fact, than the moisture content of sheathing without any exterior rigid foam.

        If you follow the standard R-value recommendations for exterior rigid foam, your sheathing will stay warm, dry, and safe.

        1. Jon_R | | #8

          If you want to call 32F and 100% humidity, "warm and dry", that's up to you.

          1. Expert Member
            Dana Dorsett | | #9

            It's the temperature averages that matter, not the peaks & lows. Vapor diffusion is a slow process. When it's 32F and 100% humidity for several hours or even a few days it's a "so what?" condition- nothing bad happens in short durations at that temp.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |