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How should I address the lack of plywood boxes for my double stud wall rough openings?

alumniu | Posted in Green Building Techniques on

Blame it on lack of clarity on my part, but when my framer built my double stud walls he didn’t oversize the window rough openings to account for 1/2″ plywood sheathing connecting the inner to the outer walls. He figured that this gap could be covered solely with drywall.

What is the disadvantage of not having plywood in this location?

I will be installing outie windows, so there will already be enough surface to mount the windows to. The backside of the inside walls have been covered with Membrain vapor retarder, and sealed with acoustical sealant and staples per instructions. I intend to blow cellulose behind the Membrain, as well as in the service cavity after electrical and plumbing rough-in. (Im hoping this will work…ive seen comments to suggest using intello plus instead of Membrain would allow for better air escape during blowing, but its too late to use that now).

I’m wondering how the lack of plywood here will impact air tightness? Should I take extra measure to apply acoustic sealant to the drywall window return?

I will need to blow in the insulation prior to drywall of course, so I will need some sort of blocker at the r.o’s in order to prevent the cellulose from flying out of this gap. Perhaps just some more Membrain stapled wall-to-wall?

Just trying to think of the implications this will have on my build.



  1. Expert Member
    Michael Maines | | #1

    Adam, Membrain will work fine for air sealing and cellulose retention. The inner and outer walls will be inclined to move independently, so it would be a good idea to tie them together with gussets above and below the window. At doors and tall windows, the gusset can go on the outside of the king stud (away from the opening). Cellulose won't fill behind the gussets so fill the gap with mineral wool or rigid foam. Membrain and Intello are equally airtight so I don't see how one could be better for cellulose installation. You can see through Intello a bit, which may help avoid voids.

  2. Expert Member
    Dana Dorsett | | #2

    Cutting in some tight fitting 1"-1.5" foil faced polyiso taped to the framing with housewrap tape to fill the gaps prior to blowing the celluose should work. MemBrain is too stretchy and would pillow out (or even rip) while dense packing.

    It's worth installing some 4" wide 1/2" ply/OSB webs every 18" or so to keep the inner & outer walls from flexing independently while blowing the cellulose and over time, which would allow you to use gypsum-board on the finish side without cracking/separating. They would of course be on the opposite side of the framing elements from the window.

  3. alumniu | | #3

    Thanks Michael and Dana, great suggestions. How should I attach webs to tie the inside and outside walls together? Are you suggesting cutting slits in the Membrain and feeding plywood rips in to attach?

  4. Expert Member
    Dana Dorsett | | #4

    Is the MemBrain already installed or something? It's better to install it after the window framing issues are resolved.

    Nail the gussets to the sides of the doubled-wall window framing, Larson Truss style:

  5. dankolbert | | #5

    How far apart are the walls?

  6. alumniu | | #6

    Dana- yes, the membrain was installed on the backside of the inside wall, and then erected and top plates will be tied together with plywood.

    Dan- the gap between the two wall layers is 4-1/2".

  7. GBA Editor
    Martin Holladay | | #7

    This is not an ideal situation. It's too bad that the MemBrain is already installed.

    I agree with the other commenters -- you certainly need some type of gussets to tie these walls together. If that means cutting the MemBrain, you'll have to cut the MemBrain, then then do your best to repair it with high-quality tape.

    Your builder's error has structural implications as well as air-barrier implications. Note to other GBA readers: Double-check your rough opening measurements before you frame your walls!

  8. alumniu | | #8

    I've seen details that suggest that the backside of the inside wall is an ideal location for a Membrain (or even plywood) vapor retarder layer. How in the world are gussets normally attached in these scenarios?

    Do you think I should toenail 2x/ply nailers to tie the rough openings together for more rigidity?

  9. charlie_sullivan | | #9

    If I'm understanding right, another issue may be the strength of the drywall surface on the bottom of the box, if someone were to sit there, for example.

    The advantage of Intello for blowing cellulose is that it's reinforced and can take the pressure. It would be better for blowing cellulose if it were air permeable, but then it wouldn't work as an air barrier. I'm worried about getting the cellulose sufficiently dense packed, which is important in a deep wall, with the mem-brain. Just to compete the picture, what is the status of the outer wall--does it have sheating on it yet, and if so, what kind?

    Lots of people have talked about that being an ideal location for the air barrier, but usually it's plywood there, not a membrane. The building science is fine with a membrane there, but as you are discovering, the construction process is tricky.

    Are you OK with the smell of the acoustical sealant? If you don't like it there are alternatives that don't smell as bad.

  10. GBA Editor
    Martin Holladay | | #10

    Q. "I've seen details that suggest that the backside of the inside wall is an ideal location for a Membrain (or even plywood) vapor retarder layer. How in the world are gussets normally attached in these scenarios?"

    A. The suggested gussets are an attempt to solve the problems arising from the fact that your framer made a mistake on your window rough openings. Lacking the usual plywood boxes at the window rough-openings -- boxes which tie the two walls together -- we are all trying to come up with an alternative way of tying your walls together. These gussets aren't necessary with most double-stud walls -- they are an attempted fix for your situation.

  11. alumniu | | #11

    Charlie- The outside walls of the house were sheathed with plywood before the inner layer of walls went up, so no access to the cavity from the outside.

    I agree with your comment that installation with the Membrain is tricky. Not only for the issue at hand, but also when the framers tipped up the last 2 inner walls they ripped off the bottom and sides of the Membrain that was so carefully attached. I had a half a day of going back and doing my best to reseal these edges with acoustical sealant and Tyvek tape. Luckily there was still some slack on the bottom, and I had left a foot at each corner to overlap, so I feel good about the repair.

    So far, the smell of acoustical sealant doesn't bother me, but that remains to be seen after I'm living it the house.

  12. alumniu | | #12

    Thanks for the clarification. I appreciate all the help and advice that you guys continue to offer.

    In regards to adding the rigidity by tying the walls together, it would be a lot easier at this point to get some 2x3's and rip down 2x4's to the necessary 2.5~3" and toenail in place around the window openings (or even drilling & screwing to avoid splitting). This way I wouldn't have to cut any of the Membrain to add gussets, and worry about detailing around them with sealant & tape.

    What do you think about this approach?

  13. GBA Editor
    Martin Holladay | | #13

    The blocking you suggest won't be quite as strong, but it will probably work.

  14. JTyler | | #14

    Have you already ordered your windows? If you are working with a company that makes the windows to your specs, you could issue a change order and reduce sizes to facilitate standard plywood connections. If you were planning on windows from a company that only offers custom sizes at a substantial up-charge, perhaps consider a custom order from a different window company?

  15. JTyler | | #15

    Maybe order some T iron and cut to lengths just shy of the width of your wall. Drill 2 holes for screws or nails at each end. Set the depth of a circular saw to match the leg of the T, and make cuts from interior to exterior every foot or so around your rough openings. Then press the leg of the T into the slots and drive 16Ds or structural screws through the drilled holes.

  16. alumniu | | #16

    I see what you're suggesting, and that would be a good solution, but I have already ordered Intus windows, and I would prefer not to undersize them to odd sizes in case I have to replace them in the future.

    I like your alternate solution as well. I think ill try ripping some 2x's first to see how that works out, but Ill keep your fix in mind.

  17. JTyler | | #17

    That makes sense. If you have any concerns about the strength of toe-nailed blocking, maybe you could drive long structural screws through the whole assembly to add some strength.

  18. dankolbert | | #18

    Intus, like many manufacturers, doesn't have standard sizes. They make them in 1/4" increments and just make them to order (thus, partly, the 14 week wait). But it might bump you to the end of the line.

  19. Dana1 | | #19

    MemBrain's vapor retardency varies with the relative humidity of the air. When it is installed just behind the wallboard the temperature of the air next to the MemBrain is nearly room temp and the RH is dramatically lower the RH of the entrained air next to the cool sheathing in winter.

    But the deeper the MemBrain is into the wall-R, the higher the RH of the entrained air proximate to the MemBrain, and the more vapor open it will be.

    In most cases putting it on the exterior side of the interior studs of a double studwall won't make it too vapor-open, but it's climate (and total-R) dependent. In very-cold climates it's important to do at least some napkin-math analysis on that to make sure it performs well enough when buried in an intermediate layer.

  20. m2akita | | #20

    Little late to this discussion, but what are the thoughts on either of the following. Both would be extra $ ( both materials and labor).

    First Option:
    Instead of wrapping your window boxes with drywall, do this with plywood with a finished, or A, surface. This could then be painted. Details for sealing your window openings might have to be a little different as you would be sealing to a finished surface. Would also have to figure out the details for the plywood/ drywall intersection on the interior side.

    Second Option:
    Connecting the two walls with 1/4" sheathing, then using 1/4" drywall on that to wrap your window boxes. I have no idea if the 1/4" sheathing would be strong/ rigid enough.

    I like Jim Tyler's suggestion of using T strapping.

  21. StollerB | | #21

    Adam- we always wrap our window (and door) openings in good-one-side plywood, and just have our drywallers run their drywall up to the edge of the RO, install a corner bead, and skim coat the entire plywood window liners when they mud their bead. You may want to brush the liners with a water and weld bond white glue solution before the drywallers start installing their corner bead, as it helps the mud adhere to the plywood properly. We have had zero issues with this. (You could just do this for the sides and top and then install a wood still on the bottom if you don't want drywall return sills too.) As long as your Intus windows leave you at least an inch of framing on your outer wall to attach these plywood liners to, you should have zero problem.

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