What is the best way to plaster an exterior wall insulated with straw bales?
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| Posted in Green Building Techniques on
We are wondering if it is wise to plaster directly on the 2″ mortar that protects the straw bales or if we should use furring and plywood on which we would fix a screen to put the plaster on. It would add an air gap between so that the wall could dry better but we don’t know if it is worth it.
What do the experts think?
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Replies
Serge,
You might want to check out the GBA article on that topic: Straw-Bale Walls.
The short answer to your question: there is no consensus on this issue. The main point everyone agrees on is that it is vital to keep the walls dry. This is best accomplished with wide roof overhangs and a foundation that keeps the bottoms of the bales well above grade.
In that article, I wrote:
"While some straw-bale builders apply plaster directly to the straw bales, Straube recommends attaching chicken wire or wire lath to the bales before plastering. “I want metal mesh in the plaster, even if it’s only light chicken-wire, for a bunch of reasons,” says Straube. “The main purpose of the mesh to keep the crack sizes small.” Chicken wire is attached to the bales using U-shaped pins poked into the bales or wires that go all the way through the bales; this is done with the help of a tool called a “bale needle.”
"Laura Bartels advises against the use of chicken wire. “We usually avoid the use of metal meshes unless required structurally,” says Bartels. “Chicken wire or stronger is required in high seismic areas, but other than those cases, you will mostly find that plaster prep involves reinforcing at corners, material transitions and over framing or other materials.”
"Traditional plastering techniques require three coats of plaster: a scratch coat, a brown coat, and a finish coat. The total thickness of the plaster is usually between 1 in. and 1 1/2 in.
"If you don’t want a stucco exterior, you can install any type of siding you want. However, it’s essential that the exterior side of the straw-bale wall be plastered, even if you later install furring strips and wood or fiber-cement siding.
"“It’s very easy to put furring strips on,” says Straube. “One way is to install a scratch coat of plaster, then install your furring strips screwed to the stucco, and then install another coat of plaster between the furring strips. A better way to do it is to install two coats of plaster and then install the furring strips, shimming as needed, with Tapcon screws. The plaster will hold the Tapcons.”
"Mark Piepkorn likes the idea of a rainscreen gap on straw-bale walls. “The general consensus of experienced and knowledgeable straw-bale builders indicates that external cladding [over a rainscreen gap] will perform better than stucco alone in nearly all circumstances, if properly implemented,” Piepkorn writes."