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What is the best way to add wool or rigid fiberglass over exterior walls and under new vinyl siding?

movinit56 | Posted in Energy Efficiency and Durability on

We live in a floating home. It is 2×3 construction, one level. Currently there is aluminum siding over very little fiberglass insulation. What is the best way to add insulation, vapor barrier and vinyl siding over the 2×3 framing? We need to keep weight down so we prefer not to build out to 2×6 wall. From what reading I have done it is all across the board as to what to add and how to avoid condensation and improve insulation and some sound barrier. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    What is your location or climate zone?

  2. movinit56 | | #2

    We live in Portland Oregon

  3. GBA Editor
    Martin Holladay | | #3

    So this is some type of a houseboat?

    First of all, your walls don't need a vapor barrier. If you are installing new vinyl siding, the usual approach would be to add a continuous layer of rigid foam on the exterior side of your wall sheathing, followed by new vinyl siding.

    One inch of EPS has an R-value of R-4, and 1 inch of polyiso has a nominal R-value of abut R-6. Two inches would be twice as good as one inch.

    If you really want to, you could install a continuous layer of mineral wool instead of rigid foam, but that approach is somewhat unusual, and you would have a harder time getting local advice if you ran into tricky areas or had installation questions. For more information on mineral wool, see Wrapping an Older House with Rock Wool Insulation.

  4. movinit56 | | #4

    Thanks for your quick reply. So to seal the windows do we do build out the window box and do self adhesive flashing over the top of the polyiso? I did read in one of the excellent articles on this site that you can leave the windows in their current location but I was not sure how you would then flash them.


  5. GBA Editor
    Martin Holladay | | #5

    If you plan to wrap your walls with exterior rigid foam or exterior mineral wool, it's essential that you understand how to assess the existing window flashing and that you understand how to flash windows. It's hard to explain every step in a Q&A column.

    Window rough openings are frequently the cause of water-entry problems that lead to wall rot.

    The first step is to assess the current situation, to see if your existing windows are allowing water to enter your walls.

    The second step is to decide whether to remove all the windows and reinstall them; or leave them in place and try to develop a way to flash the new exterior sill and new jamb extensions; or to remove the old windows and replace them with new windows.

    There are lots of resources on this web site that explain how window rough openings should be flashed. Good luck.

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