Solar Kiln Bennington VT
North Adams, MA
Hoosick Falls, NY
North Adams, MA
Solar KilnDry Your Own Wood Fast and Hassle-Free
by Dave Munkittrick
Wood is expensive. And extra-wide or figured wood is practically beyond reach. Over the 25-plus years I’ve been a professional woodworker, wood seems to have taken a cue from oil: The price keeps going up. There are ways to use less oil, but when a project requires 100 bd. ft. of walnut, you gotta buy 100 bd. ft. of walnut. That’s why I was so thrilled to discover a simple solar kiln developed by Dr. Eugene Wengert, an extension forest products specialist at the University of Wisconsin–Madison.
Wood is relatively inexpensive before it’s dry. I have managed to obtain green wood at a lumberyard at a fraction of the price of dry wood. Granted, there is a fair amount of sweat equity involved in stacking and transporting larger amounts of wood, but the savings are well worth the effort.
A solar kiln is the simplest and safest way to dry green wood quickly. Because the wood is protected from the elements, this solar kiln offers more control with much less chance of defects than air-drying provides. Unlike air-drying, it’ll dry wood to the moisture level needed for interior use. And unlike other kilns, it is designed so that it’s nearly impossible to dry 4/4 wood too fast. It’s as close as you can get to a “set it and forget it” system. After the kiln is loaded, all that’s required is some minimal vent adjusting while the wood dries. No sweating over daily drying rates and continual monitoring of the drying process.
I talked with a number of solar-kiln owners for this article. I discovered they all share one problem: where to store all their wonderful solar-dried wood.
Set It and Forget It
The beauty of this solar kiln design is that it’s almost impossible to dry 4/4 wood too quickly. Even so, most hardwoods can be dried in six weeks during the peak summer months. Our load of basswood took only four weeks. A conventional kiln dries the wood continuously and has to be monitored closely to prevent exceeding the safe drying rate for that species (see “Safe Drying Rates,” below). A solar kiln is cyclical (Fig. A, below). During the day, the kiln heats up and the fan comes on to circulate hot air through the stack. Moisture is drawn from the wood into the air and is vented outside through the vents or leaks out naturally through the kiln’s joints and seams. At night, the cooling cycle begins. The temperature drops, the fan shuts down and the moist air condenses. The surface of the wood gets wet and cool, relieving any drying stresses that built up during the day. It’s like having an automatic conditioning cycle built in.
For most 4/4 stock, it’s OK to start with the vents open an inch or two (Photo 8). This helps remove the moist air quickly. At the same time, however, you’re also letting out heat, so there is a trade-off. After the majority of the mois...