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Straw-Clay Thermal Testing

(above) The Laser Comp Heat Flow Meter Instrument is known informally at FPL as the "K-tester". "K" is the designation in the USA for thermal conductivity.

Forest Service logo

The nationally-known U.S. Forest Products Laboratory in Madison, Wisconsin has kindly offered to perform thermal testing on some samples of straw-clay material. We're very excited; to our knowledge, these will be the first such tests of straw-clay in the USA.

Bob & Larry with the Wood Miser(above) Bob Munson of Forest Products Laboratory assists Larry operating the WoodMizer saw to trim the top and bottom surfaces of each sample.

Trimming the samples is necessary to ready the them for the testing apparatus, which requires very flat, parallel surfaces for accurate thermal measurement.

(below) The samples are stacked and tagged. Before they're ready for the K-tester, however they need to fully acclimatize in FPL's humidity- and temperature--contolled room.

stack of samples

chart of German "K" values

Our samples vary in the proportions and type of staw and clay. Each sample was carefully weighed, before and after trimming, to accurately calculate density. Density makes a difference. Based on tests done in Germany in the 1980's, we expect that a less-dense, straw-rich mix will have a better R-value than a heaver mix with more clay and less straw. Our walls are on the very light -- that is, more insulating-- end of the scale.

(left) Chart showing thermal performance versus density from Franz Volhard's book on leichtlehm construction. The red oval shows the density and thermal range for very-light straw-clay."Leichtlehm" translates as "light loam", or "light clay". Examples of "heavy" construction are adobe, rammed earth and cob. As the chart shows, the conductivity of those types of walls is too great to provide enough insulation for Wisconsin's winters. With this important thermal test data, we can establish an optimum mix for straw-clay walls for a Midwestern USA climate.

View the results here

Download the PDF here (2.3 megs)

Design concepts | Site | Workshop | In-progress | House Detail | Research | Interview with Lou