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Is it possible to install rigid foam (polyiso) directly over old wood clapboard siding, without removing most of it first?

Anthony Zitelli | Posted in Green Building Techniques on

I will be removing the vinyl siding on my century old house before installing the polyiso, but unfortunately there is quite a bit of very old wood clapboard siding underneath that as well.

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  1. GBA Editor
    Martin Holladay | | #1

    I advise you to remove the old siding, for several reasons:

    1. Removing the siding will allow you to inspect the sheathing for rot or evidence of moisture problems.

    2. Removing the siding will allow you to install a layer of housewrap.

    3. You'll end up with a tighter wall (with fewer air leaks) if you remove the siding.

    That said, it is theoretically possible to proceed with the siding in place. The trickiest part would be ensuring a good air seal at the perimeter of the new polyiso. If you get air movement between the polyiso and the siding, then you'll lose the insulating value of the foam. You would have to be extremely meticulous to avoid air currents between the foam and the old siding.

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