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Community and Q&A

Closed cell or open cell foam backer rod

user-7601308 | Posted in General Questions on

Do i want closed cell or open cell backer rod for the space between my window rough opening and window jambs?

also, my rough openings are a bit oversized with almost an inch on each side, and top and bottom between RO and window jamb. Is there an oversized backer rod that would be suitable? 

And — do you install the rod on all 4 sides of the window? Do you push it in from the exterior or interior?

thanks,
forrest

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Forrest,

    What kind of windows are you installing. Fanged? flangeless? wood with brick molding?

  2. user-7601308 | | #2

    Non flanged wooden window frames.

  3. user-7601308 | | #3

    Do I only put backer rod on the sill of the rough opening? And then just insulation between the sides and top?

    1. Expert Member
      Malcolm Taylor | | #5

      Forrest,

      As Andy said: Furring down the ROs so that you have about 3/8" makes everything much easier.

      Then put the backer rod about half way towards the exterior and fill the gap with foam. You can add a bead of caulking at the interior face as further insurance.

      The sill gets treated differently. It needs a sill-pan of peel & stick membrane placed in the RO before the window is installed. If the sills are not sloped it should be turned up to create a dam at the inside face of the window frame. A bead of caulking provides the air-barrier. No insulation is used.

  4. Andy S | | #4

    "my rough openings are a bit oversized with almost an inch on each side, and top and bottom between RO and window jamb. Is there an oversized backer rod that would be suitable? "

    An inch on each side seems a bit much. I'd be furring out that RO to be a little closer to the window size. I mean, how are these windows going to securely attach with that much of a gap? How would they be trimmed out?

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