A Look at MOTHER's Solar Wood-Drying Kiln - Part One

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WE FRAME TO PLEASE

With all the construction that goes on here at MOTHER's Eco-Village, it would be prohibitively expensive to use commercial wood exclusively ...and so we figured we'd take a step in the right direction by building our own wood-drying kiln, powered by the sun, to season all our on-site and locally milled lumber in preparation for its use.

Though air drying is inexpensive and entirely suitable for seasoning construction-grade timbers, a properly designed solar kiln offers a controllable environment and also can bring a charge of wood to a considerably lower moisture content than could air alone ...an especially attractive feature to those who require more stable—such as furniture-grade—material.

The dryer we constructed has a capacity of up to 3,000 board feet of lumber with its overall floor area of 240 square feet ...though it's presently set up to condition only 1,000 feet of green wood. Actually, it's not much more than a conventionally framed storage shed equipped with integral south-facing collectors and a warm-air distribution network. Because we had originally anticipated using simple convection to induce airflow around the boards (and thus felt it necessary to place the collector surface below the stacked charge), we backed the structure into a berm to provide a gradient drop at the front. But after considering the flow restrictions that might be imposed by a full load of lumber, we opted for a fan-driven, photovoltaically powered ventilation system ...and relocated the collectors but left the berm intact.

The rear of the kiln rests on six courses of concrete block; 18 locust posts spaced on 4' centers and joined with crossbeams support the remainder of the building's 12' X 20' floor. That platform—essentially just parallel 2 X 6 joists with 3/4" plywood upper and lower skins—was filled with vermiculite because we had a surplus of that material on hand, though any type of insulation would have sufficed.

Except for the south face, the remaining three walls were framed up with 2 X 4's, sheathed with commercial siding, and stuffed with fiberglass batts. The interior plywood surface was carefully caulked at the seams and painted with white Pendex waterproof coating, which serves the dual purpose of providing a light-reflective finish and an effective vapor barrier. The roof joists, which are sheathed with 1/2" plywood and covered with cedar shakes, were supported by 2 X 4 collar ties. Three 20" X 32" operable roof vents were built near the peak of the rear, northfacing pitch.

Because the front of the kiln was to act as both a functional and structural member, it required a considerable amount of planning. We extended 2 X 6 roof joists from the peak to the top of a 32"-high stub wall located on the structure's front sill plate. That short parapet was sheathed, insulated, and fitted with seven 5-1/2" X 13-1/2" intake grilles. The joists supported fourteen 3/16" X 34" X 76" tempered glass sheets that were overlapped at their horizontal seams by 2".

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