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Community and Q&A

Ideas for insulating from the outside

ericyancey | Posted in Green Building Techniques on

We are going to be removing some wood siding from a 200 year old house. The house had blown in insulation that is going to fall out when the siding is removed. I’m considering several different options for insulation:

– batt insulation with rigid foam on the exterior, and housewrap over the foam. Issues with this are getting batt to fit properly with studs that are 18″ on center, and getting the foam/siding details correct around the old existing windows

– spray foam – could this damage existing drywall as it expands if it finds it’s way between a stud and the drywall? Some of the house is plaster, some of it plaster with drywall over it, and some of it just has drywall

I think damp spray is out because of the time it would take to dry and get the siding back on while it’s exposed to the weather.

Thanks for any help you can provide!


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  1. GBA Editor
    Martin Holladay | | #1

    If your house has no sheathing, you need to come up with a bracing plan. Once you remove the siding from a house without sheathing, you need to be sure you have a plan to (immediately) prevent the walls from racking.

    It's certainly possible to spray the empty stud bays from the exterior with spray polyurethane foam. It's also possible to install new plywood sheathing, and blow the empty stud bays full of cellulose. Then you could cover the plywood with rigid foam insulation if you wanted.

  2. ericyancey | | #2

    Thanks Martin!

    I hadn't thought about the house racking once the siding was removed - that is an excellent point. The house is balloon framed with diagonal braces in the framing - I'm sure you've seen this type of work before. Just to be safe I'll plan on putting some temporary bracing up as we go. We are only removing the siding from the North end of the house so at least the whole house won't be exposed.

    Thanks again,


  3. GBA Editor
    Martin Holladay | | #3

    If you know that the house has existing let-in bracing, that's good -- your worries are lessened. Nevertheless, if it were my house, I would install plywood sheathing or new metal strapping.

  4. Foamer | | #4


    I would go with sprayfoam, which is particularly well suited to your project. We have done several "outside - in" projects as we call them with excellent results. If you use closed cell foam, it will help stiffen your structure in addition to air sealing and insulating. Don't worry about damaging your interior wall surface; any competent installer will know how to handle that issue.

  5. jbmoyer | | #5

    I would NOT use spray foam. Martin's suggestion of cellulose and a thick layer foam sheathing (to reduce thermal bridging) is a good one.

    Spray foam contractors will try to sell you on "better performance" but open-cell spray foam has the same R-value as cellulose and blown-in fiberglass.

    AND remember you're only insulating the cavities.

    If you do a good job at air sealing your wall you don't need the expensive spray foam.

  6. ericyancey | | #6

    Thank you all for your input. Looks like I have a few things to consider!


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