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Substrate options for earthen plaster on interior over cellulose

Patrick Walshe | Posted in Green Building Techniques on

I am building a home in coastal BC and discovered clay under our plentiful sand. It would be great to use this for an earthen plaster on the interior of our house. Currently, the plan is a staggered 2×4 stud wall on a 9.25″ plate blown with cellulose. The exterior will have plywood, Typar, 2″ rigid insulation such as Roxul ComfortBoard IS strapped (perhaps with pipe or wood squash blocks or just extra thick strapping direct on plywood and Roxul cut to fit between), so that there is a rain screen gap, then fiber cement siding. The plan is to blow the wall with cellulose, but would consider packing straw into the interior stud wall (with metal mesh over?) for better earthen plaster adhesion. There is not much straw locally. Otherwise I imagine I would need to install lathe. Or would burlap or some such material work for blowing in cellulose and attaching earthen plaster. The drywall component is expensive and has high embodied energy and I prefer earthen plaster – besides why not use what we have on site?. But would it make the cellulose too wet for too long? I imagine earthen plaster would work as well as the airtight drywall approach. I would appreciate anyone’s experience and advice with this sort of approach.

Patrick

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Patrick,
    Earthen plasters are usually used on straw-bale walls, adobe walls, or cob walls, not on stud-frame walls.

    If you want to use an earthen plaster on a wood-frame wall, I think you definitely need to install rock lath on the studs before you begin plastering -- to provide a barrier between the damp plaster and the cellulose insulation, and to improve airtightness.

    If you Google "earthen plaster," you'll get a lot of results to browse through.

  2. Steven Brooks | | #2

    I'm thinking about doing something similar. What did you end up doing?

    1. Steve Knapp CZ 3A Georgia | | #4

      There are commercial products such as American Plaster. I imagine one of these companies has installation advice that you would find helpful.

  3. Tim R | | #3

    The ICC has a "Appendix R Light Straw-Clay Construction" it may provide some guidance on how to do interior clay plaster.

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