Straw Bale & Plaster Details Around a Foundation

by ziggy on August 5, 2015 -- 2 comments -- Follow

Plaster Stop at Bottom of Straw Bale Wall

Think about the transition between your straw bales and foundation early in the design process

In one of my recent posts, I talked about details at the top of the straw bale wall where it meets the roof. Now I’d like to talk about some considerations where the wall meets the foundation. This is another critical point in the straw bale wall design, and planning from the beginning of the process is essential for a clean and durable finish surface at the bottom of the wall. Here are some tips to think about regarding the plaster and details between the bales and foundation.

Details Between the Straw Bale Wall & Plaster and the Foundation

In the cabin we’re currently helping to build, we’ve incorporated an earthbag stemwall on a rubble trench foundation. (I still have yet to draw up the cross section of that particular design, but I promise I’m coming to it at some point here.) Anyway, we have an earthbag stemwall topped with a typical straw bale “toe-up”, two 2x4s running parallel along the top of the stemwall with foam board insulation in-between.

Metal Plaster Stop at Bottom of Straw Bale Wall

We install the plaster stop early in the wall building process, so we can push the bales tight up against it

This toe-up is an important feature for three reasons — it serves to lift the straw bales off the earthbags and provide a flat & regular surface, it stops any moisture that might decide to wick up through the bags (which is unlikely, but too many defenses can never hurt), and it provides a convenient nailing surface for window framing and a plaster stop.

metal-plaster-stopThe plaster stop is the detail that I’d like to talk about here, as it is an important bridge between the bottom of the straw bale wall and plaster and the finish foundation surface. In our case, the plaster stop itself is a simple piece of metal flashing, bent into an L shape and screwed into the face of the toe-up. The L catches the exterior plaster, providing a physical landing spot for the plaster and a visual transition between wall and stemwall. The L also directs water away from the top of the stemwall. Our finished stemwall will have a stone facade, but regardless of the stemwall material or finish, a plaster stop is an important consideration for any design incorporating similar techniques.

Here’s another tip if you plan to use a bent metal plaster stop. A small hem on either edge of the metal will greatly strengthen the material, and reduce any wave along the length that results from metal that only has a single bend, and no hem to strengthen it. The hem is simply about 1/4″ of the metal that’s been bent over completely at either edge.

Straw Bale Plaster Stop Detail

Bent metal plaster stop with diamond lath for plaster key — note the bamboo pinning, which we should have actually tucked behind the metal

There’s nothing fancy about how our plaster stop is attached — a few pan head screws hold it firmly in place. However, the other important detail here is the expanded meth lath (a.k.a., diamond lath or “blood lath”) installed over the vertical portion of the stop. Smooth metal has no “key” for plaster, so the lath provides a good textural surface to which the plaster can bond. This lath is also screwed directly through the plaster stop and into the toe-up.

Straw Bale Base Coat Plaster

Here’s how the wall and transition looks once base coat plaster has been applied to the bales

Note: because this metal is buried in the plaster itself, and not the straw bale wall, there’s no condensation concerns. The plaster stop is also very nice to trowel up against to get a clean visual plaster line at the bottom of the wall.

Think about how you might incorporate a plaster stop at the bottom of your straw bale wall to transition between the wall itself and the foundation. Bent metal worked well in our design, but you can also think about other materials for use as a clean and functional transition.

Here are some other related straw bale building resources:

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  • J Deguire

    This site is super informative!
    I am wondering what you are using to cover the stem walls. Will it be plastered as well or covered with another material?

    Thank you!
    J

  • Thanks! It will be faced with stone.

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