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How to construct a window buck for continuous exterior insulation with a rough opening that is too small for the plywood?

duchenyj | Posted in General Questions on

My window rough openings are 1/4″ wider than my windows per the manufacturers instructions. Problem is, I didn’t leave room to install a buck. I found this article on GBA that talks about nailing the windows directly through foam insulation:

I’m wondering if that application would work in my situation. I know it wouldn’t work with Roxul, so I’m thinking of installing foam sheathing just around the windows and nailing the windows through the foam. Is this a good idea? Are there any other options? I’d prefer not to install “innies” due to flashing concerns.

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  1. GBA Editor
    Martin Holladay | | #1

    The answer to your question depends on the thickness of the exterior foam, as well as your siding type. You need to give us more information.

    In the meantime, here's a link to an article you should read: Installing Windows In a Foam-Sheathed Wall.

    Note to GBA readers: Don't make the mistake that Jeremy made; instead, plan ahead. Make all your mistakes at the planning stage, not on the job site. This article discusses the concept: Plan Ahead For Insulation.

  2. duchenyj | | #2

    Thanks, Martin. The Roxul is 1 1/2"@ R6. The foam, if I chose to use it around the windows, would be the same 1 1/2" @R7. The siding is 1" wood, shiplap siding with a 1/2" rain screen. If I used wood for my bucks and just attached them to the studs, I thought it might contribute to thermal bridging a bit, but it would also give me an R value of 5. My walls are 9 1/2" thick. I'm adding the exterior insulation to prevent condensation from occurring into the interior of the house. Perhaps the insulative value of the wood would be enough to keep the sheathing warm?

    In my defense, I did do a lot of research ahead of time, though I did not read the article that you linked to. I'm not an east coast, west coast, guy. I'm DIY'ing my first house in northern Michigan. The builders I talk to around here don't have much experience with thick walled housing and exterior insulation with window bucks and rain screen siding. This is as new to some of them as it is to me.

  3. GBA Editor
    Martin Holladay | | #3

    The best approach is explained in the article I linked to (Installing Windows In a Foam-Sheathed Wall).

    Here is the relevant advice: "Install a picture frame of 2x lumber, with the lumber ripped from 2x6s or 2x8s. The width of the ripped lumber should equal the thickness of the rigid foam. This type of picture frame — the strongest from a structural perspective — is attached directly to the wall sheathing with long screws, with no intervening foam. This approach is the one to use if your foam is more than 6 inches thick, or if you are installing outie windows on a house with mineral wool insulation on the exterior side of the wall sheathing."

    The inner border of this picture frame should be even with the rough opening. It's true that there will be some thermal bridging through the picture frame -- but the thermal bridging won't be too significant, and in any case you have no other good way to proceed at this point.

  4. duchenyj | | #4

    Sounds great, Martin. Thanks for your help.

  5. GBA Editor
    Martin Holladay | | #5

    You wrote that you want to "prevent condensation from occurring into the interior of the house." The vulnerable surface for "condensation" -- more properly referred to as "sorption" -- is the exterior sheathing. That's where frost can accumulate. In some cases, the sheathing can get damp and stay damp for a long time.

    In your climate zone (Zone 6), you want your continuous layer of exterior insulation to have a minimum R-value of R-11.25 to prevent moisture accumulation in the sheathing (assuming you have 2x6 walls). Since you (apparently) have 9 1/2-inch studs, that minimum R-value of the continuous insulation on the exterior side of the sheathing (assuming that you want to prevent moisture accumulation in the sheathing) would be about R-20. You're installing R-6 Roxul, which is much less than R-20.

    All that said, your wall will work. Even though the mineral wool isn't thick enough to prevent moisture accumulation, the damp sheathing will dry to the exterior in the spring (because the mineral wool is vapor-permeable).

    To learn more about these concepts, see these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  6. duchenyj | | #6

    I thought the idea was that when you add insulation, this kept the sheathing warm, so that the condensation would occur to the exterior. I thought the issue was that in 2x4 construction there is enough warmth coming from the exterior to warm the sheathing, so that the cold from the outside meets the warm on the exterior of the house keeping condensation to the outside. Whereas, with these thick walls, the sheathing stays cool enough due to a lack of warmth getting through that the condensation would occur somewhere in the wall cavity. Is that not true?

    Here's another question. You mention in this article,, that when constructing outie windows, the housewrap goes to the outside of the insulation. As I am using 30lb felt this will still allow the sheathing to dry to the exterior, I guess, or would I be better off putting the felt down first? If I do put the felt over the Roxul, how do you normally attach it?

  7. GBA Editor
    Martin Holladay | | #7

    Condensation occurs on cold surfaces. The colder the sheathing, the more that it is at risk of moisture accumulation.

    Warm sheathing is happy (dry) sheathing.

    The insulation between your studs makes your sheathing cold in winter.

    The insulation on the exterior side of your sheathing makes your sheathing warm in winter.

    Lots of insulation between the studs increases risk.

    Lots of insulation on the exterior side of the sheathing decreases risk.

    You wrote, "With these thick walls, the sheathing stays cool enough due to a lack of warmth getting through that the condensation would occur somewhere in the wall cavity. Is that not true?"

    Kind of, sort of. The condensation (or sorption) happens at the first cold surface that the interior air encounters. In the case of your wall, that cold surface will be the interior face of your exterior wall sheathing.

  8. Reid Baldwin | | #8


    Are you near traverse city? I looked at some habitat houses in traverse city where they installed the windows through a layer of exterior foam. Maybe you could find one of the people involved in that project that would be willing to give you some on-site guidance. (Although the picture frame concept martin describes in #3 seems simpler.)

  9. duchenyj | | #9

    Reid, I am near Traverse. I agree, though. I'm going to go for the picture frame.

  10. Chaubenee | | #10

    Please explain what your flashing concerns are with INNIES? I found that flashing the wondows to the Drainwrap directly on the OSB was very easy. The foam is also taped adding a further layer of protection, and the sills and exterior "extension jambs offer the window a greater degree of protection from the elements.

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