Iowa Straw Bale House Innovations: Stucco Walls - Homes Across America Search Return

Iowa Straw Bale House




Goals of Innovation: Aside from the obvious goal of covering the straw bales with a protective exterior coating, the chosen material had to be low maintenance and permeable to water vapor to an acceptable degree.

Description: The information the MidAmerica Housing Partnership had on building a straw bale home stressed that an improper exterior coating could crack for a variety of reasons, leaving the bales vulnerable to moisture and mold. After reviewing the list of possible coverings with two consultants, Mark Morgan and Chris Magwood, the decision was made to use a specific mixture of stucco. It consists of 1 part cement, 1 part lime, 5 to 7 parts fine screened sand, and poly fibers to control cracking.

But stucco cannot just be smeared on a wall of straw. So, Mark supervised the volunteers who squared the walls as much as possible and wrapped the walls in a 12 gauge galvanized steel mesh. The bales were also kept in place by post and beam construction. This gave the walls the structure they needed to bear the weight of the roof. Rebar pins were driven through the lower bales and into the sills for added stability. Diamond lath around the window frames provided extra support, especially at the corners.

MAHP recruited the help of John Straube, a noted "moisture in buildings" expert from the University of Waterloo, Toronto. John studied the elements of Iowa's environment and the stucco formula developed by Chris and Mark and confirmed it was the best suited for these conditions. Two layers of "scratch" stucco were applied. These rougher coats allow the stucco to stick to the wire and adhere to itself. After allowing those to cure for two weeks, a final coat of stucco, without poly fibers, was applied, to give the home a smooth exterior. This final layer sealed any cracks that did appear.

Obstacles: Stucco concrete is basically impermeable. Our stucco mixture, however, had to address the competing concerns of exterior protection while allowing water vapor permeability. The lime we added to the mix increased its rating to 5 US perms, an acceptable permeability rating for walls. The poly fibers minimize cracking.

Cost Information: The entire stucco process, from training volunteers to final application, is estimated at $10,000.

Additional Benefits/Drawbacks: As a small rural town, Washington, Iowa, has relatively relaxed building codes. However, we wanted to hold this house to a high standard so it would be up to code in areas with more strict regulations, especially ones for "alternative" homes. Working closely with Steven Van Note, Linn County's building inspector, we incorporated all his suggestions into our "building with straw" plans. This home can meet building codes anywhere in the state.

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