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Community and Q&A

Insulate gaps in rough openings

user-6850466 | Posted in Energy Efficiency and Durability on

Would it be a good option to make rough openings 1″ or 1 1/8″ larger on each side and fill that gap with rigid foam rather than using spray foam or stuffing fibreglass in the gaps? or is there any advantage / disadvantage to this method?

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  1. Expert Member

    Rough openings are made larger than the frame size mainly to ensure you can get the door or window in and plumb it. So the resulting gap is irregular, and it would be difficult to fit strips of rigid foam in without leaving gaps.

    Having a gap that size would also make securing the doors or windows difficult. Window flanges and clips aren't typically designed to bridge that far and the less shims you need for doors the better.

    Another way to look at is is: why are you creating a large cavity you then have to worry about insulating?

    A 3/8" gap is ideal. Large enough to get a spray foam nozzle in and to hold backer-rod for caulking. Stuffing scraps of batt insulation isn't a great method, even if you air-seal the perimeter.

  2. GBA Editor
    Martin Holladay | | #2

    Malcolm did a good job of answering your questions. Your questions are a little worrisome, because it sounds as if you will be installing windows without much experience.

    You talk about "insulating" the gap between the window frame and the rough opening -- but really, air sealing is the essential need here, not insulation. So I hope you are planning your air barrier details carefully.

    Because it sounds as if this is your first building project, I'll take this opportunity to remind you of the need for sill pan flashing and good water management details. The rough opening needs to be flashed before the window is installed. For more information, see this video series: Window Sills That Won’t Rot.

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