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Will I need to install wood sheathing on my century-old house before installing rigid foam insulation panels?

adzitelli | Posted in Green Building Techniques on

Underneath the wood clapboard siding, there are only bricks and mortar in between the 2X4 studs.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Anthony,
    Your walls lack bracing. They are prevented from racking by the interior plaster and lath, and to some extent by the brick-and-mortar infill. These elements worked -- kind of, sort of, just barely -- but don't meet modern bracing requirements.

    If I were removing the old siding, I would definitely want to install a layer of plywood or OSB before installing rigid foam and new siding. The plywood layer will help reduce air leakage through your wall, especially if you tape the seams, as well as provide bracing.

    An alternative would be to install L-profile steel strapping to brace your walls. If you go that route, I suggest that you consult an engineer to be sure that you install enough strapping and have enough fasteners.

  2. Aaron Birkland | | #2

    I'm not sure you provided enough information to say definitively one way or another.

    From a structural standpoint:
    * If your house is post and beam or braced-frame construction, then wood sheathing *may not* be necessary, as the framing itself would contain bracing to counteract racking forces
    * If your house is platform or balloon construction, then you probably do need to do something to keep racking forces at bay. If for some reason you absolutely wish to avoid wood sheathing, an engineer can come up with alternatives that work for your situation, as Martin mentioned

    From a thermal/energy perspective:
    * Read this document by the building science corporation. It is possible to design your envelope using rigid foam sheathing, eschewing wood/osb
    http://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing

    Please, at least consider having an engineer or architect take a look at it. Adding wood sheathing would be a no-brainer that brings on minimal design risk. Anything else is exactly the "nonstandard" sort of situation involving the design and cost tradeoffs that engineers are born to tackle.

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