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Insulating (Interior) Gap around Window with Rockwool

jamesboris | Posted in Green Building Techniques on

I’m installing windows in a new build in Zone 2A. I plan to air seal the interior using backer rod and a bead of Prosoco Air Dam sealant. But first, I want to fill the cavity between the frameand the rough opening with insulation (like p. 9 here).

Instead of Fiberglass, like Marvin shows… I was thinking Roxul, since it won’t trap moisture (or off-gas or expand unpredictably and leave voids) like canned foam even if I pack the entire cavity. But I’ve never used rock wool before – if I buy a batt, will I be able to tear it into little pieces to stuff in there?

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you can tear off little pieces of mineral wool no problem. The only down side is you're going to have to buy a LOT more mineral wool than you need, since the smallest pack is around 40 square feet or so worth of batts. If you can find "fire pillows" in your box store, that might be a way for you to buy a smaller amount of mineral wool -- the fire pillows are essentially square plastic bags with mineral wool inside, rip open the bag and use the mineral wool for your purposes.

    The "window and door" style canned foam is better about expansion, and doesn't have off gassing issues after it's cured. I have found the Loctite "TiteFoam" and OSI's "QuadFoam" are better in some ways than the more common Dow "Greatstuff", so you might try one of those other brands. I have been particularly impressed with the Loctite product, although it does cost about twice as much as Greatstuff.


    1. jamesboris | | #4

      Hi Bill, I figure I can always use the leftovers... my wall insulation is fiberglass because I got a truckload of cutoffs from a factory here. It'll be a hassle to get the walls packed densely enough (I'm not worried about the slight R-penalty for overpacking), but free... and I made it 10 minutes before it was all about to go to the landfill. The amount of stuff that gets thrown away!

      Another issue I have with canned foam is that I feel we are all generally overconfident that we're getting a continuous bead that doesn't fill the cavity too much or too little. I think that something like Air Dam is a more reliable air seal anyway.

  2. Expert Member


    Mineral wool works fine. The only things I'd change from the detail you linked to are:

    - Do not sandwich the insulation between two backer rods. Leave a path to the exterior for any moisture that does get into there to drain or dry.

    - Form end-dams on your head flashing.

    1. jamesboris | | #3

      Hey Malcolm, thanks for chiming in. I'm not following that detail to a T or anything. That said, as far as the backer rod sandwich -- the detail shown is at the head. Typically, the head is flashed with tape on the outside, so no moisture can get out anyway. The sill is a different matter of course.

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