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Community and Q&A

Open-Cell Spray Foam for Window Installation

deerefan | Posted in General Questions on

This coming Tuesday, my window installers are scheduled to start. We are using thermally broken aluminum windows. We have covered the walls with Delta Vent SA. I have seen, in the Delta product manuals, the placement of a backer rod from the inside and the sealing it with one of their products (Delta Thaan?)

I asked one of the installers (they work for the Fleetwood window company) and he mentioned that they typically do not use a backer rod but instead spray open cell foam into the space between the window frame and rough opening. I am a little concerned about this: won’t the open cell absorb water? and how can you avoid filling the space where the water is supposed to drain?

Please advise on your thoughts regarding this situation. Thank you.

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  1. Expert Member
    Michael Maines | | #1

    Deerefan, the industry standard is to use one-part (canned) foam for the construction gap at windows and doors. It's a little denser than the open cell foam you might use in walls, but it will absorb water. It's usually not a problem but in rare cases it can be. The best approach for sealing the gap depends on the window type and other construction details. Do your windows have flanges? What is your wall assembly? What is your climate zone?

  2. deerefan | | #2


    Thank you for the quick response. The house is in Central Texas (zone 2A I believe). We are using thermally broken aluminum windows (Fleetwood). Our walls are 2x6 LVL framed with 1/2" Advantech sheathing. We will be using Cascadia clip system with 3" Rockwool Cavity Rock on the outside and 6" Rockwool Comfort Batt in the stud bay. We are using Delta Vent SA self adhering WRB on the walls with Carlisle 300 HT on the roof, all continuously taped for water and air barrier. I also plan on using Contega HF for the inside air sealing.

    Please let me know if you need any other details and thank you again for the advice.

  3. deerefan | | #3

    Is Foam really the standard? Seems like a shortcut?

    How can one prevent the gap between window and rough opening from being filled without placing a backer rod or similar?

    1. creativedestruction | | #4

      Foam is very common now for this application simply because it's quick and effective, installed from the interior maybe an inch or so deep with caulking at the exterior head and jambs. It has to be a low-expansion product specifically for sealing around window and door frames. My brother-in-law grabbed the wrong can and nearly ruined a window in my addition. It still opens much harder than it should. I've used backer rod ever since.

      Ideally you would want some thermal resistance directly in plane with the thermal break in your aluminum frames, but that's less than critical in your climate. Airtight and drained. Your Contega will do better than the foam long-term, as will acrylic tape on the exterior head and jambs. You're in good shape.

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