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| Yes,
the walls on the first floor really are made of straw - rice straw from
the Sacramento Delta that otherwise would have been burned and added to
air pollution in the Central Valley. The bales are 24 inches thick,
covered on both sides with stucco. The environment inside the sealed
wall is anaerobic, so mice and bugs are not a problem. And bale walls
are extremely fire resistant. The nominal R-value for
insulation is
R33, but tests have shown that well constructed bale walls can be up to
R50. For structural reasons, the second floor walls are stud walls but
with 2x6 rather than 2x4 studs, insulated to R20 with blown-in
insulation. The ceilings are insulated to R36. The construction method is what is called "post and beam." The wood is FSC certified or is "engineered wood" made from sawmill scraps (the beam above the window). The Santa Fe style vigas (the exposed ceiling beams) are from storm-felled yellow pine in the Sierras. |
Here are finished walls in the library, showing their curvy irregularity – part of the charm of a straw bale house – and thickness. Also seen is part of the stained concrete floor. The 5-inch-thick concrete slab provides thermal mass and thermal inertia. Like in old southwestern adobe houses, we depend on the thick walls, large overhanging eaves, and lots of thermal mass to keep the interior cool – without air conditioning! – even on 100° summer days. In a string of 8 days at and above 100°, the maximum inside temperature was 79° on the first floor and 85° on the second floor. More typical 90° summer days see maximum temperatures of 75° downstairs, 80° upstairs. Venting the house at night rapidly drops the temperatures back to the low 70s. In winter, the sun should provide nearly all our heat - using large south-facing windows to let in sun, large thermal mass to hold the sun's heat, and the high insulation value of the bale walls and R36 insulation in the ceiling. |