Building a Staw Bale House
(Page 7 of 10)
December/January 1995
By Athena and Bill Steen
The bales in the walls are pinned like load-bearing walls but they also need good solid connections to the structural supports and beams. Strips of expanded metal lath have been the most common method. Another option that has been used is to lay ladder-like sections of durowall, on top of each or every other course of bales and then pinning or tying it to the bales below. Sections of threaded rod can also be inserted through the posts and out the other side of the bale wall where they can be secured with seismic plate washers.
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It was once commonly thought that infill methods automatically meant more lumber consumption than load-bearing buildings. One clever design, which has been called a modified post-and-beam, has reversed that thinking by using less lumber than a duplicate building that had been built as a load-bearing structure. It came about by adding extensions to the window and door bucks that were com monly being used with load-bearing buildings so that they could serve as the structural supports in combination with 4x4 posts at the corners. These were connected to a small-dimension, glue-lam beam, which saved considerable lumber in comparison with the traditional loadbearing roof-plate.
The in-fill approach affords greater design flexibility for larger structures with complex floor plans. Multiple stories, complicated roof configurations, and large amounts of glazing are possible. The structure can be roofed before the bales go in, and if the bales in the wall were to become damaged by moisture and lose their integrity, they could be replaced without affecting the structure of the building. Bale in-fill approaches are typically more acceptable to code officials and can make building permits easier to attain. Prescriptive standards for in-fill bale construction are included in the Tucson and Pima county codes and in-fill code language has been developed for the state of New Mexico and is waiting approval. Transverse load tests have also been successfully conducted in New Mexico as part of their code approval efforts.
Hybrid Straw Bale Options
Hybrid bale structures have begun to borrow from both the in-fill and load-bearing systems. Standalone structures require that columns or posts have sufficient thickness and strength to resist buckling. However, very small dimensional lumber like 1x2s sticks, or bamboo, when braced against the exterior and interior surfaces of the bale walls, can have sufficient strength to be structural.
In Mexico, where the bales were of such poor quality, or where the walls are extremely tall and long, we have used bamboo or rebar in this same way to add incredible strength and stability to the otherwise precarious load-bearing bale walls.
Wall Finishes
The majority of straw bale buildings in the United States have been finished with cement stucco, while many in Canada and France have been finished with traditional lime plasters. Gypsum plasters have also been popular for interior applications, as have earth plasters. Earth can also serve as a very effective exterior finish depending on the conditions, climate, and stabilizers.
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