Adapting a Greenhouse for a Solar Kiln

Thoughts on converting a polycarbonate-glazed greenhouse to use it as a wood drying kiln. April 18, 2009

I have a few questions. I have a 12' by16' greenhouse with a gable roof, made of all double layer polycarbonite panels. Can I use this for a solar kiln in the Chicago area? Also, can it be done in the winter? Tell me if this is true. Someone told me that I could set up a kiln with a plastic tarp, a dehumidifier, a fan and an electric baseboard heater in my heated shop? I have about 1500 bd ft of quarter sawn oak to dry.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Yes, you can make the greenhouse into a solar heated kiln, adding fans and so on. You probably want to also add some absorber panels so that solar transmission losses are minimal.

You can build a dehumidifer kiln in your shop, but be aware of the safety issues (from fire for example) and that you also need to have fans. You also have an insect risk. Using a home-type DH unit will work, but the unit will likely be destroyed due to the acids in the oak. Also, 1500 BF of green red oak will have a lot of water that must be removed - about 4000 pints and that must be removed carefully to avoid cracking the wood. To evaporate this moisture you will need about 8 to 10 million BTUs of heat energy, although the DH unit will supply some of this.

From contributor Z:
Most green houses set on bare ground, assuming this is the case, won't that be an RH concern?

From Gene Wengert, forum technical advisor:
Yes, it would be a concern if moisture can move through the ground. The floor needs a moisture barrier and also should be insulated, especially at the edges. Likewise, opaque walls need to be insulated as we want to get peak temperatures 30 F above outdoors.

From the original questioner:
The greenhouse is sitting up on a stack of 6x6's above the ground 6" in the front and 24" in the back the right side wall and roof is facing due south. The inside of the 6x6's is filled with gravel and a treated deck was built on top for the floor. Can I lay plywood on the floor or do I still need to build a platform and insulate it? Should I also frame walls and insulate too? If so, basically I am building a solar kiln inside the greenhouse, should I leave the front wall open or should I build a kneewall and insulate? If this is the case I might as well just build a solar kiln and not take up space in the greenhouse.

From Gene Wengert, forum technical advisor:
Without the insulation , the interior will not get hot enough. I do believe you need to insulate the floor as well as use a plastic vapor barrier under the plywood. Usually we do insulate the south wall (vertical) as the heat loss is too high compared to the small solar gain. Likewise, insulate the knee walls. So, you are indeed building a solar kiln inside the building. Incidentally, a solar kiln, or any kiln, is a piece of equipment and is depreciated (tax-wise) in an appropriate manner.

From Gene Wengert, forum technical advisor:
The reason that wood will not dry is that the fans are run too long. Only run the fans when the sun is shining and the kiln is about 20 F warmer than outside. If fans are run 18-24 hours a day, it is hard to get under 10% MC.