SPENDING THE DAY WITH STRAW-CLAY INFILL
Last July, Doug and I joined the timber-raising of one the carpenters who helped build Underhill House. (Check it out in my post Timberframing with Friends: Sweat, Love and a Little Drama. )
The addition is being insulated with straw clay infill. This is a great substance. We’ve visited several homes made this way and participated in a straw clay workshop a few years ago. Straw traps air in the wall and creates a good thermal barrier. The clay hardens fluffy straw into a solid block and provides some thermal mass. The materials are inexpensive, and natural. The skills required are easy to learn in a few minutes. However, everyone I know who has built in straw clay (this includes me) agree that straw clay infill is very hard work. That’s why Doug and I went over this Tuesday to lend a hand.
Prairie has made a lot of progress since July. He’s got the roof on and everything framed in. He had begun the straw clay process, finishing the bottom of the south-facing wall. That’s the easy one, it’s going to be almost half windows in the best passive solar design.
Preparing the Clay Slip
Prairie is using a watering tank to mix the clay slip, a concoction of clay and water mixed together into something that resembles a chocolate shake. When left for a few days, the clay particles sink, so Doug’s first task was to mix it back up again using a mixing paddle on a power drill.
Tossing the Straw and Clay Together
Each straw needs to be very lightly coated with this mixture. Prairie was working with a ratio of about 4-1/2 gallons for each straw bale. There are many ways to combine the slip and the straw. We were working with the basic, fluff-it-with-your-arms method, which gets the job done and is a full-body workout besides. It’s like tossing a huge straw salad until every single piece is lightly coated in clay dressing.
TAMPING AND STAMPING
Straw clay infill is then transformed from a sloppy pile of clay-coated straw on the floor to a solid wall by being molded in a temporary frame. Each bale/batch was gobbled up as soon as it could be mixed. At first the frames were close to the ground, and it was easy to climb in and compact the mixture by foot. We tamped into every little corner and crevice with wooden sticks to make sure every square inch was filled uniformly. That is a LOT of tamping as well as stamping.
We got a good rhythm going with Prairie building frames as fast as Doug, I, Issac and Todd could mix up straw and clay and pack it in. When we reached the top of the first frame, Prairie was ready with the next.
We called it a day as we all began to feel like our quality control might be starting to slip. By that time – even though it had been raining all day, Prairie was already able to remove the bottom frames, and we got to assess our work. There are always a few soft spots that missed compaction, and those can be filled from the outside with a mixture of straw clay that is a little more wet with clay. It is far from dry, but will hold its shape now and dry faster without the frame.
The outside will be sided with wood, and the inside will be plastered with earthen clay. It’s so satisfying to watch an old farm house in a beautiful setting get a new lease on life, with the kind of sustainable materials and a sense of community that no doubt built it in the first place.
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