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Community and Q&A

Rigid foam with faced fiberglass?

Stephen Atkins | Posted in Energy Efficiency and Durability on

My existing walls have faced fiberglass insulation (in 2×6 framing). Zone 5a. I’d like to put 2″ of rigid foam on the outside of the sheathing when I re-side. I know this is recommended with no vapor barrier on the inside. Will the facing on the fiberglass prevent drying to the inside too much?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Will the facing on the fiberglass prevent drying to the inside too much?"

    A. No. Go right ahead and install the rigid foam.

  2. Stephen Atkins | | #2

    Thank you. After looking more closely, my existing walls actually have un-faced fiberglass with a plastic sheet probably stapled across the studs after the fiberglass was installed. The original paint may have been oil-based. Does this change the situation?

  3. GBA Editor
    Martin Holladay | | #3

    Unfortunately, it does. The poly is not useful, and to some extent makes the rigid foam addition more risky.

    However, many people had added exterior rigid foam on houses with interior poly without failures or problems. The key seems to be: are there any signs of moisture problems in the existing house? If all evidence points to dry walls and ceilings, without mold, water stains, or dripping, you are probably okay. If there is any way to poke a few holes in poly, that wouldn't hurt.

  4. Stephen Atkins | | #4

    Thank you.

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