Many modern-day straw bale builders used to cover their bales with metal wire before plastering. This technique originated from the plastering of cinder blocks, which needed the texture from the metal wire to hold the plaster in place. Since straw bales have more than enough texture to hold the plaster, we felt it was a waste of time, money and resources to use this technique. Therefore we plastered our bales directly. At the time we did this (1996) we were considered quite radical; luckily things have changed in the bale building world and adding unnecessary chicken wire to plaster friendly bales is now seen by most builders as unnecessary. Not surprisingly, our new building was also plastered directly onto the straw bales; this is now considered a standard practice in Straw Bale building. How nice it is to have one’s radical methods become the norm!
We plastered the exterior walls first, because they were more at risk for moisture damage. Once we finished, we immediately plastered the interior walls, because if one side remained unplastered for a long time the walls would settle unevenly.
The house is plastered almost entirely with lime and sand plaster, which was applied by hand, by master plasterers. We were lucky to find two men, both in their 70s, who had been plastering all their lives. They were planning on retiring but they were intrigued by our project, they had never plastered with a pure lime plaster and they had never heard of straw bale houses. After plastering our house they went on to have quite a career plastering straw bale houses all over our area- retirement was out of the question!
We chose lime plaster because it’s breathable, won’t hold in moisture, and resists the growth of mold and mildew. On the exterior walls we used 3 layers of lime plaster. On the interior walls, we applied one layer of our own mud plaster followed by two layers of lime plaster. If we hadn’t used a layer of mud plaster, we would’ve needed an additional layer of lime plaster. Since 3 layers of lime plaster were not essential on the more protected interior walls, we opted for a layer of mud plaster, a truly local product we were able to create by hand from materials right in our backyard.
Lime plastered straw bale walls also function as a sort of air exchanger. Through these breathable walls, fresh outside air can travel in and inside air can travel out, while the unique and mysterious thermal properties of the plastered straw bale walls help prevent the loss of inside heat. This air exchange helps purify the air and moderate humidity. Under ordinary conditions, we have found an air filter unnecessary, and we have never had any moisture problems in our house-even in the bathroom which does not have a vent fan. We feel that there is something about the combination of wood, straw and lime plaster that helps to moderate indoor humidity levels; we find that our house is moister than other homes during the winter, and dryer during humid periods.
Our new building was plastered by Andy Mueller of Greenspace Collaborative and Andy’s methods are slightly different than what we did the first time around. For the first coat Andy uses a clay, lime and chopped straw plaster blown on with a plaster sprayer. We sourced clay from a local clay pit and had that trucked the four miles to our house. Once the exterior and interior coats of clay/lime plaster are dry, Andy trowels on a second coat of lime, sand and finely chopped straw (lower proportion) plaster, similar to the one we used on our original house. This second coat is the final coat and is followed by a coat of whitewash. Because curing lime plaster is weakened by freezing temperatures, the second coat of plaster was not applied to the new building before winter 2008; it will be applied in Spring 2009 when it will be followed by whitewash.