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Community and Q&A

Windows: interior sealing

Jamie K | Posted in General Questions on

– Andersen* (window manufacturer) says: fill interior 1/3 of cavity w low expanding foam and then sealant on top; do not overfill or will result in poor performance
– DuPont says: do not use foam at sill or 6” up jambs from sill (probably where vapor closed flexwrap NF was using as flashing); then sealant on top.
– Insulator says: Great Stuff spray foam all around window
– 475 says: do not use spray foam anywhere and especially not at windows, use only mineral wool

1) Good practice to use Great Stuff spray foam then something like OSI Quad Max caulk overtop?
2) But window manufacture and DuPont say for some reason Not to use too much foam and/or Not to use Any foam at sill at up jambs 6” – why is this?  What should we do?

* windows are Andersen Fibrex windows

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  1. User avatar
    Michael Maines | | #1

    I'm curious what Andersen's reasoning is for only filling the cavity 1/3. I imagine it's because many, perhaps all of us who have worked in the field have overfilled cavities, resulting in bowed jambs. DuPont's concerns may be because one-part foam can absorb a bit of water, and/or it won't allow for sill pan drainage for a leaky assembly. The most important thing is to stop air leaks and to control water vapor movement; 475's foam-free approach works, in conjunction with their vapor-permeable tapes. My approach is typically to fill the cavity with one-part foam, in two or three lifts so I don't bow the jambs. I usually allow the sill pans to drain by spray foaming only the interior portion below the window sill. But each window manufacturer has their own instructions, and it's a good idea to follow them if you want any hope of a warranty.

    1. Malcolm Taylor | | #2

      "Each window manufacturer has their own instructions, and it's a good idea to follow them if you want any hope of a warranty."

      I'm not a lawyer, but I wonder about that. Wouldn't the warranty denial need to be materially linked to the defect or failure? I can't see a court siding with a manufacturer who tried to deny a claim for a failure completely unrelated to the installation not exactly following some small instruction in a long list supplied in the installation manual. Especially when identical products from different manufacturers have quite different instructions.

    2. Jamie K | | #4

      Malcolm, I'm not as interested in window warranty as much as I am having it not fail in the first place. If it should fail and they cover it, great - if not, at least I did my best trying to do a good job both following how they suggested to do it as designed better where I thought it made more sense to it another way that made more sense. There are so many loop holes for manufacturers to get out of warranties, I'm not as interested in fighting but getting it right to begin with.

    3. Jamie K | | #5

      Hey, thanks for your thoughts, Michael!

      - Is Great Stuff spray foam considered one-part foam? As opposed to closed cell spray foam, which we will be using to spray our 2 ft above grade concrete stem wall.
      - You mean even once it's dry spray foam can absorb a bit of water? Possibly this is why DuPont does not want it on "interior" side of sill & 6" up jambs so that the drainage effect is not allowing moisture to get sucked into the dried spray foam as the flashed drainage installation method is trying to allow it to go down and out of the sloped sill pan. Water should never reach the amount of interior sprayed foam behind flanged window, if I'm thinking right?

      We already have windows installed and DuPont flashed with drainage method and sloped sills created out of beveled cedar siding (that I got I believe from a suggestion in one of your articles). I believe what I'm hearing is that you don't see any problem using Great Stuff spray foam all the way around the "inside" perimeter of the window as long as it is not over filled (functionality wise for air/water/vapor)?
      Then, we can probably just add a light layer of interior caulk over it..
      [This is what Andersen recommends albeit 1/3 filled probably meaning not overfilled like you said.]

  2. Deleted | | #3


  3. User avatar GBA Editor
    Martin Holladay | | #6

    Q. "Is Great Stuff spray foam considered one-part foam?"

    A. Yes.

    Q. "As opposed to closed cell spray foam?"

    A. Sort of. It would be more accurate to say, "as opposed to two-component spray foam" -- the type of spray foam used by spray foam contractors (and homeowners who buy two-component spray foam kits).

    Q. "Even once it's dry spray foam can absorb a bit of water?"

    A. A bit, although the amount depends in part on the type of spray foam. Open-cell spray foam can hold more water than closed-cell spray foam.

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