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Cob on straw bale walls

Posts: 7
Location: zone 10 B, S. Florida
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We want to use cob over the straw bale walls.  Would that work well in central AZ?  It will be in Yavapai County.  Thank you!
Posts: 34
Location: Jacksonville, OR
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Hi Syrena,

I say you posted something yesterday about this too--I'll address both posts here.  

Cob is a mixture of earth soil that contains clay and sand, mixed with some straw (and water to make it workable), and is usually applied in handfuls.  Earth plasters are really quite similar--same materials but somewhat different material ratios, perhaps a little bit wetter so they can be applied with a trowel, and generally applied in thinner layers.  Earth plasters (clay is the binder) are fabulous!  If you dig the earth from the building site or nearby it will have a very low embodied energy, and are usually quite easy to work with.  There's a pretty good chance we're talking about the same thing when you say cob and I say earth plaster?

Both the IRC Appendix S Strawbale Construction Code and the CASBA book use the term "clay plaster" when describing any plaster that has clay as a binder, and earth plasters do.  Earth plasters will have other material in them like silt, organic matter, and some aggregate, too.

The building code requires that clay plasters over straw bale walls be at least 1" thick, and must be applied in at least two coats. (The code with its very informative commentary is printed in Chapter 7 of the book).

Because plasters are quite heavy--one square foot of 1" thick plaster weighs 15 lbs.--there's a limit to how much the fuzzy straw bale surface "lath" can support without being supplemented with plastic or fiberglass netting or some other bio lath attached to the bales that will help support and bear the heavier, thicker plaster layers.  Some builders will use 17 gauge or heavier (thicker) galvanized wire mesh, but others feel this could eventually rust in an earth plaster.  (Note: Please don't use "chicken wire!"  Much of the remodel work I do on older straw bale buildings stems from plaster failures caused by the use of chicken wire as a lath instead of 17-gauge stucco netting or heavier gauge mesh. Chicken wire is generally only 22 gauge or lighter (meant to protect chickens, not support plaster), and it has a thinner galvanic coating that more readily rusts.)   Because of this weight issue, the code limits clay plasters to 2" thick, which the commentary tells us is mostly a concern in seismic areas, and can be exceeded if the building code official determines that a thicker plaster doesn't present a significant risk (of falling during an earth quake!).  The commentary notes that thicker can be beneficial, in terms of thermal mass.

If you plan to use earth (clay) plasters on the exterior, be sure to design for them. i.e. protect them from the weather.  Wind-driven rain will erode an earth plaster, and even when it has been coated with linseed oil, lime wash, or potassium silicate, these treatments must be renewed periodically. There's a reason why cultures that used earth plasters on their buildings have a tradition of annual maintenance!  If you are building in an area where wind-driven rain is typical, or your preferred design has tall, vulnerable walls or doesn't have large protective roof overhangs, consider lime plaster for the exterior as it's more durable.  

On the inside of a building, there's nothing better than earth plaster for creative expression, ease of repair, or finish color options.  Chapters 2 and 6 of the books describe in greater detail the many considerations for designing for and installing interior and exterior plasters.
S. Carpenter
Posts: 7
Location: zone 10 B, S. Florida
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Thank you, so much!  That is very helpful information.  

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