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Can I install polyiso continuous insulation over existing painted wood siding?

Aaron Stout | Posted in Green Building Techniques on

I am quoting a siding job on a small house on a lake in central New Hampshire, climate zone 6. They get a lot of weather and wind off of the lake. From the inside, the house currently has 1/2″ drywall, 2×6 stud framing (I believe it’s 16″ layout), fiberglass insulation (probably faced), 1/2″ OSB sheathing, WRB, and ship-lap or t&g wood siding installed vertically (I believe it’s pine) which is painted with latex. I would like to install at least 1″ of foil-faced polyiso foam board over the existing flat siding to act as an air, water, and vapor barrier. Then I would install a rain screen mesh or battens and wood siding, to be pre-finished or painted. The windows and doors would be retroactively flashed to the outside surface of the polyiso and then extended with tapered subsill and extension jambs. Does this scheme cause any problems?

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  1. Aaron Stout | | #1

    I have also been thinking about using rock wool instead of polyiso. I have seen Matt Risinger talk about it a lot on the Build Show on YouTube, but I haven't researched the implications yet. And of course there are other foam options. Please advise!

  2. Expert Member
    Akos | | #2


    Its hard to get a continuous air barrier over shiplap. Probably doable over T&G with a peel and stick membrane. Either way, it is probably more work than stripping the siding off. Something like shiplap takes no time to strip and you gain an extra 1" of space so less trim work needed.

    In zone 6 you should be installing about R12 of exterior insulation. You can go with less since there is an interior vapor retarder, but in that case I would go for a more permeable foam as it is a safer assembly. Unfaced EPS or permeable poyiso (ie IKO ener-air) are much better choice.

    Mineral wool is expensive and lot of work to get flat, make sure you price it before committing.

    With exterior insulation retrofit, the best is to keep your WRB under the foam, so it is easy to integrate with the existing windows and doors flashing.

    1. Aaron Stout | | #3

      @Akos, why is it hard to get a continuous air barrier over shiplap (the existing siding is square matched)? Wouldn't foil-faced polyiso with taped seams double as an air barrier (and triple as a moisture barrier)?

      1. Expert Member
        Akos | | #4


        I was thinking about clapboard. Shiplap should be fine. Foil faced foam can be your air barrier, with the siding though there could be a lot of paths for air leaks which will be hard to seal up. Much easier to wrap the whole house in peel and stick, which could also serve as your WRB under the foam.

        Since foil faced foam is impermeable, more robust assembly is to install at least R12.

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