A few weeks ago I was at a natural building workshop a couple hours north of me on straw bale house building. I’ve mentioned it to several people, and their reaction has been along the lines of: “Straw? You mean like the 3 little pigs and the big bad wolf?” I concede that three pigs would have been successful with their straw house if they had only included wire and plaster on either side of the straw!
Straw bale building is an interesting building option. I’ve been learning about it over the last year and am coming to the realization it’s one of the best building materials around. (Along with that was getting some hands-on experience – and hence, the workshop.) Bale buildings can be completely sustainable, possess fantastic insulation properties, high fire resistance, and natural aesthetic appeal. Straw bale houses can last for hundreds of years with little major maintenance if built correctly. I like it particularly because it is simple to build; and yet can be cost effective both in construction and operation. Remodeling is not quite as easy as a standard house, but usually it isn’t too difficult.
A short history on straw bale houses in North America can be found here: http://thesustainablehome.net/the-original-nebraska-straw-bale-buildings/
There are two main types of straw bale: load bearing and post-and-beam. In load-bearing, the bales themselves take the load from the roof; whereas in post-and-beam the roof load is taken up by the framing. This home was a new type of load-bearing.
The owner brought in a truck load of rice straw bales from NorCal. You can use other types of bales, but rice generally makes the best. In foreground on the above picture are pallets of natural hydraulic lime (NHL) plaster. This is special stuff imported from St. Astier, France and has been mined there since the Roman times. Generally, you need a lime plaster for straw bale for its high permeability. You can use concrete stucco, but the permeability of concrete is generally too low and can cause moisture and mold problems later on.
Early Monday morning, there was a slab with two 4×4 sill plates bolted down; nothing else. We worked 10 and 12 hr days most of the week to get the house ready for the building inspector to sign off on the shear and rough electrical / plumbing inspection on Thursday. Then we started the exterior plastering on the last day.
The carpentry team arrived and began putting up these rectangular boxes for the walls that we proceeded to fill with straw. The boxes help to guide the walls, contain the straw, and provide transverse shear resistance. The straw itself is supposed to be stuffed into the boxes and help carry the weight of the roof. From an engineering perspective, I’m not completely sold on this approach, but that’s what was there…
Part of the simplicity of straw bale is that the wall exterior and interior surfaces are nearly the same. Once they are covered with mesh, the mesh is “sewn” together thru the straw using a long needle and bailing twine. This helps hold the mesh onto the straw and prevents excessive sagging while applying the plaster.
Shaping the window opening is where the most skill is required to get a uniform surface. Once you get the hang of it, it’s not too hard. Significantly over-filling the area with loose straw and then pulling the wire mesh taunt over the straw gave the best results for me.
The scratch coat provides the standard shear resistance for the wall, so it dries for about a week prior to the roof being installed. The architect told me that a finished plaster possesses nearly equivalent shear resistance to a standard OSB shear panel on a per foot basis. In this type of load bearing structure, the vertical roof loads are mostly supported by the plaster. Seems a dubious design practice to me.
Overall, it was a lot of work but I’m thankful to have been able to go and learn. I’ve come around to liking the strawbale building concept, so I’m keeping it in mind for the future!
