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Double Foil-Faced EPS for Exterior Insulation

Chuck Ellis | Posted in Energy Efficiency and Durability on

I have an opportunity to get 50 sheets of 2 inch 4×8 foil on both side insulation boards really cheap. Doing a total remodel on a 60 year old home with a 40 year old addition. There is some fiberglass insulation between 2×4 studs but not sure how much or where. Do I need to be concerned about vapor transfer in lieu of the foil on the insulation. Will be installing house wrap, a 3/4 inch nailer for rain screen, and then probably hardi plank siding. Zone 4

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  1. Expert Member
    Zephyr7 | | #2

    A vapor barrier will often CAUSE wood rot, not prevent it. Just adding a vapor barrier isn’t a solution to much, especially if done without any planning. You can easily cause more problems than you had when you started.

    As long as your wall can dry to the interior, you should be fine. Just make sure to don’t have interior side polyethylene or vinyl wall paper. The exterior foil faced EPS is already a vapor barrier, unless it’s perforated (and even then, 2” thick EPS isn’t very vapor open), so adding anything else on the outside of that EPS to do anything with vapor is moot. A rain screen would be a good idea though.

    Bill

  2. Jonny_H | | #1

    Yeah, don't listen to this guy: https://www.greenbuildingadvisor.com/article/r-value-scammers-sued-by-the-ftc

    To actually answer your question, I think you'll be fine, as long as there isn't a "class 1 vapor retarder" on the inside of your wall (class 1 would be something like polyethylene sheet, something else foil-faced, or vinyl wall covering). Look at table 2a / 2b in this report: https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf

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