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Community and Q&A

Fiberglass insulation and spray foam around windows.

TIMGABRIEL | Posted in General Questions on

Would it be advisable to leave fiberglass insulation then fill remaining void with spray foam? In the event fiberglass got wet, would it ever dry if spray foam filled the remaining void? What would be the best option for insulating widows? Thank you for reviewing my question.

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  1. Expert Member
    Dana Dorsett | | #1

    If it's easy to take out it would best to install backer rod and fill it up with an appropriate can foam, but it's not super critical. If you just compress the fiberglass until it's pretty firm but still has some give, and fill the rest with can foam it should be fine. The fiberglass would have to dry toward the opposite end of the mini-cavity, but in most cases there would be an adequate path.

  2. GBA Editor
    Martin Holladay | | #2

    If you've done a good job of flashing, the fiberglass shouldn't get wet. As Dana wrote, it should be able to dry out even if it does get wet.

    It's usually best to skip the fiberglass insulation. Canned spray foam is the usual material for providing an air seal and a little bit of R-value, although many installers improve the installation with interior air sealing tape.

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