Students erect a cordwood wall during a workshop at Earthwood Building School in West Chazy, N.Y. A center air space and cavity between the log end courses is filled with sawdust, adding to the thermal performance of the wall.
Earthship Biotecture has recognized the need for additional insulation to prevent wide temperature swings and heat loss, especially in cold, cloudy climates. Designers now recommend an uninterrupted sandwich of insulation board outside the tire wall—typically a pair of 2-inch-thick sheets achieving at least R-14—on all but the south side of the building.
This so-called thermal wrap approach has other design elements aimed at thermal stability. The insulation board is set between a volume of mass at least 4 ft. tall and 4 ft. wide. Inside, the north/ south depth is no more than 20 ft., to balance the mass volume and living space volume.
Correct tire stacking is critical to avoiding failures in load-bearing walls. Earthship has developed its own tire building code, aimed at both building inspectors and owners. The voluntary code lays out the correct sizes and coursing directions for bearing and retaining walls. For instance: Walls over 6 ft. high must have a ground course of 15 inch or larger tires.
The code also details how to fill the tires with earth: All casings must be packed to 90% compaction with a 6-9 lb. sledge hammer. The code also sets minimum guidelines for plates and bond beams supporting roofs.
“It’s very standardized,” says Kirsten Jacobsen, Earthship’s communications director. “If you didn’t know they were tires, they look like building blocks.”
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