How to solar kiln dry your lumber

by Rob

Well, I could go into a really long speech about designing and building your own solar kiln. Then an even longer speech about the actual drying process. But there are more in depth articles already on the internet written by experts in the lumber industry. So I will just give you a quick overview and point the way to everything you need to know about solar kilns, solar kiln plans, and the process of solar kiln drying. Enjoy!

Here are my views on solar kilns.

1. They are extremely easy to operate with out messing up your lumber. If you understand the principles of drying lumber.

2. They are extremely cheap to build and operate. I have about $400 into the construction of mine. The operating costs are only the electric that is used to run the fans for air circulation.

3. The process of heating during the day and the cooling at night, results in minimal stresses in the lumber. Drying to fast and to harsh will result in case hardening, checks, splits, and warp in the lumber. The solar kiln minimizes this, just by the way it works.

4. You can buy rough-cut green lumber from sawmills for a fraction of the cost of surfaced and dryed lumber from a distributor. So, you can buy cheap lumber, dry it yourself and save your money for more shop toys tools!

5. The only bad thing about solar kilns is that you have to wait for your lumber to dry, instead of buying dryed lumber right when you need it.

Here is the quick explanation of building a solar kiln. If you want to go way in depth in solar kilns, then read the articles provided in the links. But for you impatient guys, I give you the jist of it.

The main things to do are,

1. Make sure the roof faces south for us in the Northern Hemisphere.

2. Make sure the walls and floor are insulated very well and very complete. You need the high temps during the day.

3. Use a good glazing for the roof. A non UV blocking transparent material is what is needed. Corrugated Poly carbonate from a green house supply company is the best thing to use, but even plastic sheeting will work for a short time.

The first things to consider when getting ready to build your solar kiln are where are you gonna put it, and how much lumber do you want to dry at a time?

For a small solar kiln, that will hold 100-250bft of lumber, you will need an area approximately 5'+ wide and up to 12-16' long, just for your kiln to sit. And if you live in the northern hemisphere, you will need an un-obstructed exposure to the south, and visa-versa for the southern hemisphere.

The roof of the kiln will be covered with a "glazing" (poly carbonate green house panels are best for a roof, but anything transparent that will allow the UV rays to penetrate into the kiln will work fine.) I used painter's plastic sheeting from Home depot and it worked fine for a summer, then the UV rays had broken it down and it began to crack and rip.

The roof should be pitched at roughly the same angle as your latitude, I went 45 degrees here in NY, and my Lat. is about 47, and it worked fine.

The floor of the kiln can be a concrete pad or a regular joist type floor. When I built my solar kiln, I laid two white oak 4"x6"x8' beams on the ground and leveled them. I built my kiln on skids so that I could move it, if I ever needed too. I then laid a painted sheet of 3/4" plywood on top of the skids and then put 2"x6" floor joists down and insulated between them. Then another piece of 3/4" plywood, painted with aluminum roofing paint or something else to seal it and keep the moisture out, is laid on top of the joists.

After the floor is down, you can begin erecting the walls. I used 2"x4"s for the studs and again, insulate well. Then paint all exposed surfaces inside the kiln, flat black for maximum heat absorption. I used the first design shown here. You need to leave two vent openings at the top of the back wall and two at the bottom (approximately 1' square each). Then cover them with screen to keep pests out. Then build some doors to close off the air flow into the vents went needed.

*NOTE* I built it like it is shown, with the door on the end. This makes it hard to load and unload the kiln. I had to build a small cart and platform, so I could pile the lumber on the cart and then just push the whole pile into the kiln. If you do not want to do this, I recommend putting the door on the back wall instead.

Now you need to mount the circulation fans in the top of the kiln. I used metal attic fans from Home Depot, you just have to remember that the temperatures in the kiln can reach up to 200+ degrees in the peak during the day, so you need fans that won't melt. They will suck in fresh air from the top vents and blow the air down the roof slope and through the stickered lumber pile, and finally out the bottom vents. Must of the time the vents will be closed and the fans will just circulate the air through the lumber pile.

After roughly a month and a half in the summertime, your lumber will be dryed. For more in depth information refer to the following articles. They will explain every little detail you could imagine.

Great information and many different designs

Solar Kiln Designs 1 -- Solar Heated, Lumber Dry Kiln Designs - Part 1

Solar Kiln Designs 2 -- Solar Heated, Lumber Dry Kiln Designs - Part 2

Solar Kiln Designs 3 -- Solar Heated, Lumber Dry Kiln Designs - Part 3

Solar Kiln Designs 4 -- Solar Heated, Lumber Dry Kiln Designs - Part 4

Other sites about drying lumber

Lumber Drying: An Overview of Current Processes pdf (Excellent Info)

A CHECKLIST FOR DRYING SMALL AMOUNTS OF LUMBER pdf

Working With Wood I. Home Drying Lumber

TIMBER SEASONING IN AUSTRALIA

Super long article on drying. I think you need to be a rocket scientist to understand it. But just incase...here it is anyway. pdf (long download time)