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Air-Sealing Windows on Interior Side

Ryan_SLC | Posted in General Questions on

Recommended or your practice for best product on air sealing new windows?

Hey all,

I’m putting my nail fin windows in today. I think I have the outside understood okay at this point.

Once the exterior is all finished…I’m not sure what is best for the perimeter sealing on the interior.

I understand some of the window frame is covered by whatever is the trim, but it would seem pretty insignificant…so the idea of tape is confusing me.

Any gold standard product or method? For example, if it’s Siga tape, which version? Or if it’s a caulk, which brand/version? I’m at a total 0 for knowledge.

Thank you so much!

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  1. freyr_design | | #1

    Wigluv is a pretty standard for air sealing interior of windows. If you already liquid flashed sill I would use that with backer rod. As far as coverage, all windows are a bit different in width but just make sure you have reasonable coverage ( some tapes may have specs). With liquid flash I think 1/4” is probably plenty. Just remember you can always add blocking and shims on interior RO to make your trim line up perfect with your window and make sure it covers tape/LF (same if you are considering drywall returns). Make sure your window reveal is close to even on the window as your eye will pick up slight offsets.

    1. Ryan_SLC | | #2

      wonderful. Thank you so much for your thoughts!

    2. tim_william | | #4

      Why wouldn't backer rod and caulk suffice to air seal the space around a window? If you go by most manufacturer RO instructions, the space will be ~1/4" all the way around.

      1. freyr_design | | #5

        Who said it wouldn’t? Liquid flash is basically caulk, just a high performance caulk. The benefit of liquid flash over your standard caulk is the elasticity over time and thicker install. I’m not sure if that was the question?

        I personally prefer the liquid flash over tape as I think it’s easier to get perfect, just painter tape the window and install LF and you have seamless air/water barrier. Just a little messy….

        Edit: also, to clarify, there are a lot of different kinds of caulk and I would not use certain ones, primarily I wouldn’t use acrylic caulks as the have a tendency to harden and crack, especially over a backer rod. Silicone caulk could work but you have to be more careful oh the shape and thickness of application. You can look up recommended gap thicknesses and profiles if you google it.

        1. Ryan_SLC | | #6

          Hey thank you for the response.

          On liquid flash, I'm thinking of the Benjamin Obdyke and Prosoco I used for the rough ins. Is this the same product you'd look for, or is it different in this context?

          Thank you so much!

          1. freyr_design | | #7

            Ya though with your odyke luck I’d probably go with prosoco. I haven’t looked at their air dam tech sheet but I imagine it’s very close to their LF with maybe a couple different additives.

            My personal preference is the Henry LF airbloc as it’s generally cheaper and I’ve had good luck with it. Prosoco might be a little high perm but I don’t think it really matters in the grand scheme of things and last time I look it was 30% cheaper.

            Edit: Oh also I would get myself some exam gloves and just use my thumb or finger to smooth it out as you can get a really nice bevel, just like with normal caulk. You can then clean up on ro with putty knife

        2. tim_william | | #8

          Thanks for the response, my question was more along the line of "why overcomplicate this detail with fancy tapes and such?"

          1. freyr_design | | #9

            oh sorry I misinterpreted, agreed with sentiment

  2. Ryan_SLC | | #3

    Does Prosoco AirDam receive much attention compared to tape around these parts?

  3. andy_ | | #10

    I know the title said "best" product, and in this case is a small number of windows, but since we're talking about sealing the inside of windows maybe a discussion about what's truly needed? What's wrong with the simpler and more affordable approach of backer rod and Big Stretch caulk?
    What's wrong with a little canned window and door foam covered by caulk?
    Is an expensive tape or liquid flash really needed here? Or are we doing this for bragging rights?

    1. freyr_design | | #11

      It’s not actually that much, if it all, cheaper. Especially depending on where you get it. If it’s about the same price, might as well use the nicer product.

    2. Ryan_SLC | | #13

      I like your comment Andy. This site has definitely opened to my eyes of "right" "cheap" and poorly done.

      I'm game for best performance, where performance is air stopped.

  4. nrosdal | | #12

    backer rod and air dam from prosoco is what i did and it worked great (i did run out of air dam and that is a special order around here so the last few windows were done with big stretch and it accomplished the same thing but the prosoco does feel way more flexible and robust). I cant imagine why someone (expecially if you used a fluid applied on the window framing) would go with a tape here unless the gaps are huge or the person is trying to save 5-10min per window at the cost of a less likely to near 100% seal.

    1. Ryan_SLC | | #14

      Thanks for the opinion. I appreciate it.

      One last question on it all. It might sound kinda dumb, but I'm not a pro.

      Is window/door less expanding foam or backer rod easier to work with or the preferred option? Fundamentally the same-thing in concept, but I have no experience to assume one is easier or does it better than the other.

      Thank you!

  5. Ryan_SLC | | #15

    To answer my own question, there are a lot of threads on here about the possible too great expansion of foam and then that single part spray can deteriorate. I've read nothing that backer rod is known to deteriorate.

    I'll go with backer rod and ordered a sausage of Prosoco AirDam to go over the backer rod.

    Thanks all!

  6. cal_egan | | #16

    Siga Meltel is also a great caulk for this application and it comes in white and black.

    We’ve had the best results pulling the painters tape off about 15-20 minutes after applying the caulking. It comes off super clean. If you wait for the caulk to fully cure it can be a bear.

    1. Ryan_SLC | | #19

      Unlike other Siga stuff, it's actually priced lower than other stuff. Dang it.

      Thanks for the lead. I would have tried the Siga Meltel if I hadn't already bought Prosoco AirDam. Just for giggles.

  7. tjanson | | #17

    I've been using backer rod and DAP Dynaflex 230. It's tedious, but I prefer it to filling the cavity with spray foam and inhibiting drainage if there is a leak into the RO.

  8. jackofalltrades777 | | #18

    On an ICF wall with an anchor bracket/recessed install. I used closed cell spray foam (window/door formula) between rough opening, SIGA Fentrim Grey tape on the outside, then placed 1/2"EPS foam on top for synthetic waterproof stucco attachment. On the inside, I used SIGA WigLuv tape. Tedious and time consuming but never had a leak yet.

    I'm not a fan of standard "caulks" as they will eventually crack and open up over time due to thermal expansion of the window frames and wall structure. The SIGA tape stretches and applies to a greater area of the window and rough opening, which should outlast the life of the building.

    1. Ryan_SLC | | #20

      I didn't even think of tape having that benefit. As long as it remains stuck and doesn't pull off...okay. I can see that as a benefit. Since it has a width to it I can see the benefit that you'd never assume it had a leak on the outside. Only actual failure could happen where it "stuck" to the window...but even then it's still got a structure to it. Good point.

      Without thinking or hearing that part, I on my own turned down the idea of tape for the same reason you said--horribly tedious.

      I think you made a good point for a benefit with tape. Thanks for the comment

      1. Expert Member
        MALCOLM TAYLOR | | #21


        Christine Williamson advocates for leaving the outer half of the jambs and head open, both as a drainage path, and to equalize pressure with the outside. If you want to use foam you just push a backer rod to where you want it to end and fill from there.

        My own feeling is a lot of this comes from creating the cavities in the first place. I prefer to carefully frame and square the RO, so that once the window is in there is a 1/4" gap at the jambs and head. That small gap is incidental in terms of a thermal bridge. Not worth insulating, and easily air-sealed with tape, or backer rod and flexible caulk. Of course that strategy doesn't work when you can't rely on the framing.

        1. Ryan_SLC | | #22

          Thanks Malcolm for the thoughts.

          I wish I had noticed, but one of my 3x5 window rough ins was undersized. I had to hack it up pretty good to get the window to fit.

          Live and learn, but a perfect rough...yep. That would save all conversation and questions :)

  9. Ryan_SLC | | #23

    So I didn't take my own advice. I've used Great Stuff plenty-o-times...but I got cute and quick and did one window fast with it. Hours later, yep, my window/door Great Stuff leaked out. User error and should have known.

    Just slowly did my three other windows with backer rod. Back rod is the answer after having done both. No questions. do I get the yellow off my white vinyl frames?

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