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Community and Q&A

Windows’ Rough Opening Too Wide? (How to fill?)

matt2021 | Posted in General Questions on

In the extension room we have been building (from a pre-existing porch), we are installing Marvin Elevate windows.  When framing the walls and the openings for the windows, we followed the rough opening (RO) measurements given by Marvin.  So, if a window has a RO of 37″, we made sure there were 37″ between the side studs.  I don’t know why Marvin gives such indications, which leave 1″ (one full inch!) on each side of the window.  So, now, for the two windows that have already been installed, there is an inch, even an 1 1/4″ that I must find a way to fill.  With the windows already installed, adding, say, a half inch strip of plywood does not seem easy (though maybe I could just glue it with liquid nails?).

Spray foam is actually dis-recommended by Marvin.

I’d like to fill the gaps with backer rod and sealant, as recommended by Matt Risinger in one of his videos.  Backer rod wider than 3/4″-1″ is not easy to find.  (I can order online, but also, I guess, I could get around that problem by using pipe insulation?)  What worries me is having a bead of sealant, over the backer rod/pipe insulation that be 1″-1 1/4″ wide.  Is that acceptable?  Or does it make more sense, for the two windows that were already installed, to glue 1/2″ plywood (weather resistant?) or some other 1/2″-3/4″ lumber with something like liquid nails, and then to use a much thinner backer rod and sealant for the remaining gap?  I could think of other ways to fill the gap, including inserting a 1″ strip of rigid foam, and then caulk with sealant around that; but I am curious to hear what folks think would be a good approach.

For the other window openings, where we have not yet installed the windows, I think I will add a 1/2″ of zip sheathing to the sides of the window opening prior to installing the window; Steven Baczek likes to do that, I have noticed.  I think that a 1/2″ gap on the sides, while making inserting the window into the opening not as easy, would still be sufficient, and would give me a tighter fit.

If it matters, for the room, I’m using ZIP System R-6, and ZIP StretchTape and Flashing Tape as flashing for the window openings.

Thank you very much for any suggestions!

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  1. Expert Member
    Michael Maines | | #1

    Marvin Elevate specs rough openings 1" larger than the frame size, or 1/2" all around, not 1" all around. My guess is that they had enough complaints from people who couldn't get the rough opening plumb and level that they added extra space. European windows are usually sized for 1/2" construction gaps as well. As a designer, I no longer specify rough openings, I specify the frame size in case the builder ends up changing the window brand.

    If your construction gap is 1/2", I would use a backer rod and sealant. If it's larger than 1/2", I would furr the opening in with rips of framing lumber or plywood.

    1. matt2021 | | #5

      Thanks, Michael! Maybe I should add some pictures to better explain the situation. There is a 1/2” gap around the widest part of the window, that is, near the exterior. Yet, there is a 1” or more where the jam is.

      The situation, it now turns out, confuses the contractor (who does not seem to know much about windows, unfortunately). He thinks that, with the 1/2” between the sheathing and the window, filled only by a 1/2” of the nailing fin, the window is not stable enough so to say. He wants to put some wooden “blocking” that, by being against he narrower part of the window, more to the interior, but before the jam, the nailing fin will be prevented from flexing inward. I don’t know whether that’s a real issue. That half inch gap is what the manufacturer recommends. Yet, since we are going to fill the wider gap, towards the interior, with wood, if he uses a 3/4” thick board, he should also get that “blocking” effect.

    2. matt2021 | | #10

      Dear Michael,

      Could I call your attention to the follow-up questions I posted below, to Malcolm? As I explain there — as you mention above — Marvin has the rough opening specifications so as to have a 1/2” gap all around. Yet, past the widest part of the window, that is, once the window narrows at the jambs, by another 1/2”, the gap becomes wider. So, I’m thinking of applying wood just for the window jamb area, so as not to interfere with the ease of installation the 1/2” gap provides. My contractor seems to think that, without some “stoppers”, the window will not be firmly in place. Isn’t the nailing fin designed to support the window even if there is a 1/2” of the fin that does not rest on the studs? I have to admit that, if pushed, the window moves. Is that normal?

      1. Expert Member
        Michael Maines | | #11

        Matt, do you have interior extension jambs? Those often step in from the structural frame. I rarely use them. The actual window jamb is the part that should have close to 1/2" gap. If it's a lot more, somebody was sloppy with the carpentry.

        1. matt2021 | | #12

          Thanks, Michael. We have regular jambs, for 4” walls. Since I’m using the ZIP R system, my walls are thicker; so, we will be extending the jambs.

          Did you have a chance of seeing the picture of the window? A 37” window calls for 37” RO, as per Marvin’s guidelines. And the carpenters created openings that are 37” wide. The actual size of the window (that is, the widest part of the window, right under the nailing fins, is 36”). That’s the part in white. Yet, past that, the window becomes narrower, by a half inch all around. That’s the part that, in the picture, is not painted. Those are the jambs, right? The width of the window, there, is 34 3/4”. That’s why, on the interior, the 37” RO creates the wide gap.

          1. Expert Member
            Michael Maines | | #14

            Matt, yes, here is a screen shot of the last project I did using Elevates. (The actual project has triple-glazed windows but Marvin doesn't or didn't have drawings for them.)

            I had forgotten about their two-piece construction. I would seal the outer part with backer rod and sealant, or tape. That part of the window is where the sash is--it's the thermal control layer, the air control layer and the water control layer all in one, and those layers should be continuous with the same control layers at the rest of the building envelope. On my project below, the window is installed on an exterior buck for cost reasons but there is a small energy penalty as a result.

            The interior jambs aren't doing much. You can stuff some fiberglass or mineral wool into them if you want.

  2. kbentley57 | | #2

    If you've got a full inch on each side of the window, you've got plenty of room to slot in a piece of 1x4 as you said. It's not structural, it only serves the same purpose as the backer rod, without being backer rod.

    1. matt2021 | | #6

      Thanks! For the window that has already been installed, such a wooden board could just be attached with liquid nails you think?

  3. Expert Member


    With a full 1" gap, did you have enough backing to nail the flange in securely?

    1. matt2021 | | #7

      Thanks Malcolm! I think the nailing fin is securely attached, as there is only a 1/2” gap around. It’s only towards the interior, where the window jam is, that the gap is wider. (Does this explanation make sense to you? Maybe I should take some pictures.)

    2. matt2021 | | #9

      Hi again, Malcolm. I’m attaching some pictures, with two follow-up questions.

      1) If we install a wood board on the sides and maybe the top, from the interior (and for all the other windows PRIOR to the windows being installed) up to where the window widens, i.e., we leave the 1/2” gap between the window at its widest point and the wall, the 1/2” cavity behind the nailing fin will remain empty and inaccessible because of the wooden board. Provided that the nailing fin is all nicely flashed from the outside, is such a gap a problem? For the windows that have not yet been installed, I could try to glue a 1/2” backer rod to the edge of the wooden board before installing the window; that way, the gap would be a little fuller. But I’m not sure I see the point of doing any of this.

      2) If I add a 1/2” or 3/4” board on the sides and top, just on the more interior side, there will be a 1/2” or 1/4” gap remaining, between the board and the window jamb, which I would be filling with backer rod and then caulk. Personally, I would prefer adding a 1/2” board, so as to have a 1/2” space to fill with backer rod and then caulk. Is a smaller or wider gap preferable you think?

      3) My contractor believes that it would be good to have some sort of wooden “stoppers” for the window: pieces or wood that, by touching the surface where the window gets closer, would stop it from moving inward. In other words, he seems not to trust the nailing fin’s ability to hold the window firmly in place, given that there that 1/2” gap all around. Yet, the 1/2” gap comes from Marvin’s instructions. So, is there really a problem with the nailing fin not being strong enough? The window that was installed, if one tries to push it outwards, for example, I must say does move a bit. And that’s the nailing fin moving. Is all this normal? (Of course, those stoppers the contractor wants to install would not prevent the outward movement; yet, he really thinks he can mount the window better and more easily that way. Personally, I think he underestimates how having such blocks might interfere with the process of getting the window all square and plumb.)

  4. briancornwell | | #4

    Prosoco Air Dam.
    Fill with Advantech, glue then tape.
    Spray foam, low expansion in 1" max strips.

    1. matt2021 | | #8

      Thank you, Brian, I’ll definitely look into the Prosoco Air Dam product!

  5. jackofalltrades777 | | #13

    CLOSED-Cell backer rod (Amazon online they sell 1") & then SIGA building tape on outside and inside window frames. Problem solved.

    1. matt2021 | | #17

      Thanks! On the outside, I’ll have ZIP flashing, since I’m using ZIP R6 sheathing. On the inside, the tape would be difficult if I were to hope to apply it to the 1/2” gap (see pictures I posted). I think a 3/4” backer rod squeezed into the 1/2” gap, and then sealant, would be best. Past that, however, where I have the 1” gap (where the window has its jambs and is narrower — as that’s what Marvin windows look like — stuffing some fluffy insulation might be best. Prior to installing the drywall, though, I’m tempted to apply Siga tape, as an additional air barrier. What do you think of that? And, in case, which Siga tape? The wigluv 20/40? Or is Siga Rissan 60 a better choice?

      Also, if you think that taping that 1/2” gap, reaching it through the 1” gap, might not be too difficult, please correct me, on the option of using sealant instead. I just fear that these tapes are going to be super sticky and difficult to apply in a narrow cavity. Also, aren’t they supposed to be pressed, to activate the glue? That would not be an option in that narrow cavity.

  6. matt2021 | | #15

    @Michael Maines, thank you! So, that means I should NOT add wood to reduce the gap where the jambs are, or the part that really matters will not be easily accessible, if accessible at all. Is that right?

    Unfortunately, I’m left with the contractor’s belief that some “stoppers” against which the “step” from the sash part to the jamb part would rest, would help (if not, he seems to think, be essential). He thinks that, because the nailing fin has a 1/2” portion that is not against the stud, the window will not be quite solidly installed. He thinks that the pvc nailing fin flexes because of that 1/2” gap. Should I just tell him, “Don’t worry, this is fine; it’s my house, it works this way”. I’ve noticed, by the way, that has the caulk hardened, the window that has already been installed no longer flexes that much.

    1. Expert Member
      Michael Maines | | #16

      Matt, I can't recall actually installing Marvin Elevates myself and I try to be careful about giving advice on things I haven't done myself. I have read through their installation instructions many times but it's been a while and I can't recall the details. In general, though, I'd agree with you; adding solid blocking behind the jamb means you won't be able to seal the window where I think it's most important, at the plastic outer portion. However, I would bet that the vast majority of these windows are fully foamed into place, and the next most common approach is probably to seal the interior edge of the jamb as your builder plans to do.

      I would consult the installation instructions, and if you're still unsure of what to do, call their technical hotline--you'll likely get someone nice and knowledgeable on the phone who has answered your question many times.

      1. matt2021 | | #18

        Thanks Michael! I’ll call Marvin tomorrow. That’s an excellent idea.

        The idea of adding wooden boards was mine, to fill the wider gap, where you say I can just put fluffy insulation. My contractor wants wooden blocks (or a continuous board), to make the window installation stronger. Yet, with such blocks, plumb of the window will be constrained by where the blocks have been installed, I think. And I’m skeptical that the installation recommended by the manufacturer were to lead to an unstable installation.

        (As for myself, I had incorrectly speculated that the area right behind the nailing fin could have been left empty. So, thank you SO much for explaining how that would have neglected the most important area.)

      2. matt2021 | | #19

        Michael, I spoke with Marvin’s tech support, and they were very helpful, as you predicted. They confirmed that the part to seal is the area right behind the nailing flange, as you had explained. I can use one bead of spray foam, or backer rod and sealant — they say. I like the backer rod plus sealant option better, in terms of sealing effectiveness. In terms of solidity of the assembly, I’m not sure; maybe the spray foam will keep the window more firmly in place.

        The areas near the jambs can be filled with loose insulation — they said.

        My carpenter still insists on using some blocks — just a few — because they will help him get the window plumb, he says. I’m not going to let him to do that. I want the whole gap open, so as to be able to properly seal. And squaring and plumbing the window, I think, will be a matter of spending enough time during the installation, with one or two people outside, and one inside checking levels during the process.

        1. Patrick_OSullivan | | #21

          > My carpenter still insists on using some blocks — just a few — because they will help him get the window plumb, he says. I’m not going to let him to do that.

          You often need to shim windows, so I don't think your carpenter is being unreasonable with this. This is part of the reason why the method that I find to work really well is:

          1. Extend your air barrier to the inner edge of the window. (If using sheathing as an air barrier, practically this means turning the sheathing into the RO or taping from sheathing into the RO.)
          2. Install the window. Shim to your heart's content.
          3. Cut shims flush with jamb.
          4. Use low expansion spray foam on all four sides.
          5. Trim spray foam flush to jamb.
          6. Apply sealant over foam (and the cut shims), contiguous from RO to jamb.

          In this setup, you're effectively using low expansion spray foam as backer rod and you end up with a flush surface that's easy to seal, regardless of shims.

          My favorite sealant and foam for this process is Contega HF and OSI QUAD foam.

          1. matt2021 | | #22

            Thank you, Patrick, for the suggestions and, also, the product tips! My carpenter does not quite want to shim; to that I would have no objection; indeed, I've encouraged him to shim more than he has, on the first two windows that were installed. He thinks that wooden blocks (maybe three or four per side) nailed to the rough opening, will help with getting the window level and plumb, and that such blocks will prevent the window from moving inward, towards the interior of the room, when applying screws into the ZIP R6 sheathing (as he thinks that the foam of the ZIP sheathing gets compressed by the screws). I told him that, using screws, he will have control, and should not overdrive the screws. And that, when installing the windows, we should move at a small pace: install a couple of screws, and then have a second person check for level and plumb from the interior, shim if needed, and install another couple of screws. Then, when happy with the install, work with foam/backer rod and sealant, and move on to the next window. Proceeding with caution and at a not-too-fast pace seems preferable to me to installing blocks that will be on the way when trying to insulate, and that might still not quite be in the right place in terms of the window alignment.

            That being said, his insistence, on that being a stronger assembly, makes me wonder: maybe a block here and there, provided that I can still insulate and seal properly, would help the window never move inwards, as the sash portion (see pictures) would be in contact with such blocks?

          2. Expert Member
            Michael Maines | | #23

            Matt, I was picturing the blocks as inside the construction gap, but if I understand correctly, your carpenter actually wants to install them on the interior side of the window? In that case, they could be installed with screws to do what he wants them to, then removed so you can do what you want to do.

          3. Patrick_OSullivan | | #27

            > He thinks that wooden blocks (maybe three or four per side) nailed to the rough opening, will help with getting the window level and plumb, and that such blocks will prevent the window from moving inward, towards the interior of the room, when applying screws into the ZIP R6 sheathing (as he thinks that the foam of the ZIP sheathing gets compressed by the screws).

            I am now so thoroughly confused by his suggestion, I don't know quite what to say other than my other comment that any reasonable window installation process does not compress Zip R6 to any noticeable degree.

            One thing that is good practice (with any sheathing, really) after cutting the sheathing RO is to thoroughly nail the sheathing border to the framing RO. Sometimes you will see the sheathing "suck in" when you do this, but that's just planing it out to where it's supposed to be, not distorting it.

            If he is really compressing the polyiso to the degree being implied, I'd love to see it, but also would be surprised that the sheathing itself isn't compromised.

  7. Deleted | | #20


  8. matt2021 | | #24

    @Michael, he wants to install the blocks on the side of the rough opening, so that the sash part of the window would be resting against them. So, they would be in space that there is between the jamb and the stud. So, unfortunately, they would not be removable. Now, if they really help as he thinks (or help him, as he is not acquainted with this type of window), I could leg him install the blocks, and go around them to reach the 1/2” gap between the sash and the stud, and maybe seal with sealant around those blocks. It would make insulating and sealing harder, but still doable. I just wonder why we should do that, given that Marvin does not have any recommendations in that sense. His reason has to do with the wall, at the exterior, not being fully flat (because he hammered that ZIP R6 sheathing, in certain spots, way more than it was needed) and the possibility that the very operation of installing the windows, hence installing screws through the nailing flange, will, again, compress the ZIP R6 insulation.

    Maybe he could nail (with finish nails) small portions of composite shim in the spots where the ZIP sheathing is a bit compressed and creates a little step with the bordering ZIP shearing right where the window flange must go (as it’s the strip of sheathing that is between two windows), prior to flashing it with tape?

    1. Patrick_OSullivan | | #26

      > the possibility that the very operation of installing the windows, hence installing screws through the nailing flange, will, again, compress the ZIP R6 insulation.

      I have 30+ windows installed on Zip R6 sheathing. There is no normal installation process that should compress it.

      1. matt2021 | | #28

        Thanks, Patrick! Indeed, I think that a more gentle, careful, mindful approach would have not caused any of the damage to the sheathing, which unfortunately has occurred. Now, for the installation of the windows, I will monitor them very, very carefully. They will be using screws, not nails (hence no more blows); the screws will be long enough to penetrate the studs (something that might not even be needed, if I’m not wrong); there will be no reason to overdrive those screws.

        I still wonder about the areas where the ZIP R6 sheathing has been compressed and, hence, is uneven: shall I order some ZIP Liquid flashing, and level the areas that way? (Just a thought) Or should I not worry too much about that?

        1. Patrick_OSullivan | | #29

          I don't think I'd try to screed liquid flash across my sheathing. That would get very expensive very quickly!

          However, as alluded to in my other reply, what do the nail holes look like? I just am having trouble envisioning a fastener that can compress the polyiso but not also pull through the sheathing in a compromising fashion.

          1. matt2021 | | #30

            Thanks, Patrick. Sorry for not having caught your point, and sorry for not being clear about the damages. The real damages are:

            - some nails that have penetrated the sheathing too much; (I’m planning to check that the sheathing is still properly fastened, and tape the hole or put a smear of liquid flashing);

            - areas where the carpenter hit the sheathing with his hammer (to drive a nail that was sticking out), and managed to compress the foam;

            - areas (maybe only one) where the sheathing actually cracked.

            The latter two are extremely aggravating to me. Unfortunately, the folks working on my house, it turns out, are clueless about this type or sheathing. I don’t have an alternative to working with them, at this point; I can only repair the damage and try to reduce the chances of any further damage.

            Among the future “damages” I can try to minimize there are those that might derive from misconceptions. One of them is this persuasion the carpenter has, that the screws we will be using to install the windows will compress the sheathing’s foam. His reasoning must be: a screw that pushes the nailing fin against the sheathing, and that is well secured into the stud, will squeeze the foam between the sheathing and the stud. I’m not sure that will happen; and if it were to happen, it would be because of overdriving the screws. I intend to monitor things so that he doesn’t do that.

            What he doesn’t seem to realize is that where the sheathing’s foam is compressed, or the sheathing even broken, it’s because of hammer blows. Where the nails have penetrated the sheathing too much, it’s because of the nailer being too strong. That’s my layman’s diagnosis.

          2. Expert Member
            Michael Maines | | #31

            Matt, you have my sympathies. As a designer I have worked with builders like that and my solutions were to either start my own construction company or to avoid working with that builder again.

  9. Deleted | | #25


  10. matt2021 | | #32

    @Michael Maines I never thanked you for your sympathy. It’s appropriate to the situation, unfortunately.

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