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

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The south face of the wood-drying kiln is covered with recycled tempered glass.

When you pay a visit to your local lumberyard, the wood you take home will have more than likely been dried in a gas-powered wood-drying kiln. These sophisticated cookers can process commercial lumber by the tens of thousands of board feet at a clip. However, Mother Nature offers an excellent source of energy that ultimately can accomplish the same task and will allow you to produce your own lumber, from forest to finished board, at a cost your local dealer couldn’t come close to.

On the other hand, drying is tricky; it’s easy to get wood to shed moisture, but it’s another thing entirely to control the process so that the resulting lumber is usable. The last thing you’d want to do is simply lay your green boards in the sun to bake.

Why? Because the dampness in wood exists in two forms: bound water, which is captured in the cell walls, and free water, which is held in the cell cavities. The goal in seasoning is to bring the wood to a moisture content (MC) — designated, by percentage, as the ratio of the total weight of water in a given amount of wood to the weight of the sample when it’s been completely oven-dried — compatible with the dampness of its environment. This is known as the equilibrium moisture content (EMC), and it varies with the surrounding air’s relative humidity.

Simple air drying removes the free water, which accounts for the wood’s moisture content above 30%, or so. Below that fiber saturation point, natural evaporation occurs more slowly, since the wood must then give up its bound water. And the release of this cell wall moisture can give woodsmiths fits; it causes the cells to shrink, resulting in stresses that can warp or damage the finished product.

Now, shrinkage always accompanies drying, but uneven shrinkage creates problems. Wood, as you might suspect, dries from its surface inward. Hence, an imbalance is created between the high-moisture core and the lower-moisture exterior, which causes the water to move toward the surface, where it evaporates.

Too rapid or uncontrolled moisture removal shrinks the cells at the surface, preventing the interior moisture from escaping properly through the outer shell. The stress created can cause a variety of defects, including honeycombing (internal collapse), case hardening (simultaneous compression and tension in the same slab), warping, checking, and splitting.

Conversely, if the drying process is too slow, conditions may become ripe for the development of fungi that cause mold and stains, spoiling the appearance of the wood.

We Frame to Please

With all the construction that goes on here at MOTHER EARTH NEWS’ Eco-Village, it would be prohibitively expensive to use commercial wood exclusively, 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 material, such as for furniture.

The dryer we constructed (refer to our Solar Kiln Diagram) 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. We 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, north-facing 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″.

Understandably, some daylight is needed inside, but the wood shouldn’t be exposed directly to the sun’s rays. By the same token, solar energy was needed in the form of heat, so we harnessed it in both ways by covering the back surface of the joist framing with black-painted aluminum press plate, from the collar ties down to a point halfway to the stub wall. A barrier consisting of I” foil-faced polyisocyanurate was extended from the base of that wall to a horizontal sill strip 68″ below the aluminum plate, and the open space between was covered with more press plate and 16″-wide sheets of fiberglass-reinforced plastic glazing placed at eye level.

Essentially, this arrangement permits an airflow from the intake grilles to pass through the cavity between the stub wall and the foil-covered insulation board and up through the channels between the joists, gathering warmth as it moves. At the same time, it blocks most of the sunlight while letting the absorber plate heat up. (In fact, the aluminum sometimes has a tendency to transfer too much heat into the structure by radiation. We could solve that problem by insulating the rear surface of the plates, thus forcing the thermal energy to remain in the channels.)

To deliver the tempered air to the stacks of wood, we constructed a 13″ x 15″ horizontal plenum of insulation board at the upper terminus of the roof-joist channels, and routed four sections of 6″ insulated duct tube between it and a second vertical air shaft built onto the rear wall. Five lengths of capped 6″ perforated ABS drainpipe were likewise used to distribute the air from the base of this box to the stack piers, which were simply a series of concrete blocks.

Because the kiln isn’t located near any power source, we decided to let the sun drive the air as well as temper it. One 12-volt, 35-watt ARCO photovoltaic panel provides the energy needed to operate a 12″ fan within the vertical plenum. Since the PV panel is wired directly to the fan motor, the air delivery peaks out in fall sunlight at 350 cubic feet per minute. (One panel can handle about 1,000 board feet of wood. To increase the kiln’s drying capacity, we’d have to add the appropriate number of panels.) Unfortunately, the periodic stagnation that occurs on overcast or inclement days is conducive to the development of mold in especially moist, fresh-cut lumber. If that turns out to be a chronic problem, we’ll alleviate it by installing a storage battery and charge controller in the circuit.

Preparation Is Imperative

The success of each seasoning depends, in part, on the manner in which the green wood is prepared and stacked. Ideally, the raw material should be end-sealed immediately after being cut. This is done by applying a latex paint to the exposed end grain and covering that, when dry, with an oil-base coat. If the wood is to be stored prior to drying, it must be kept in a ventilated shed, and the boards must be stacked on sturdy supports — with air space and stickers between each one — to allow proper circulation. If this air-drying stage is eliminated, the lumber can be loaded right into the kiln after sealing, but it’s still imperative that the pieces be properly ticked. We place each board 2″ from the next, and separate the layers with 1×1 pine stickers. Finally, each stack of material (a full charge consists of two stacks) is wrapped in poly at the sides, with the top left open. This allows the tempered drying air to work its way completely through each pile, with a minimum of leakage. The roof vents can then be adjusted to regulate the flow. (Our kiln differs from many in this feature: the stacks have their own microenvironment within the rest of the structure.)

How does it work? Well, we’re still monitoring its performance with a Lignomat wood-moisture meter and a hygrometer, though we’ve already been rewarded with a supply of finished lumber (we were also taught a lesson in the importance of proper stacking when part of a charge mildewed because of inadequate and uneven airflow). A new batch — 500 board feet of oak and an equal amount of yellow pine — is undergoing tests right now. In MOTHER’s Solar Wood-Drying Kiln: Part Two we’ll examine moisture levels, analyze the 60-day drying cycle, and determine if the quality of the finished product in fact warrants our expenditure of $1,000 or so in building materials and $360 in electrical components. We’re not about to champion our design without data, because the proof of success — or disappointment — is in the data.