Solar kilns - putting it all together

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Compiling and analyzing information on the efficient design and construction of a solar kiln. November 22, 2003

I'm trying to gel all the info on this site regarding the best design and construction of a solar kiln.

If I have things correct:
a) heat storage isn't an issue;
b) double-insulated collectors certainly help;
c) R-20 or better insulation on the walls is necessary;
d) protect the structure from humidity and condensation!;
e) adequate cfm through the stack is a must;
f) regular monitoring of RH and temp (ventilation) is important in order to achieve proper drying and quality wood.

So, as I put it together:
1) To use solar panels from a big-box store is acceptable. The fancy double-insulated stuff from a solar supplier isn't necessary.
2) Use only CDX, no OSB. Seal any CDX edges or joints on the kiln interior by taping them with 4" cotton fabric coated with tar. Coat the joint and embed the fabric. Then, lightly overcoat the fabric.
3) Butyl rubber latex caulk also works well as a joint-sealer.

4) Fiber bats aren't good for insulation, but the pink foam stuff (closed-cell) is.
5) Krona (brand) insulation sheeting, taped together with 3M 8951, works great on the inside. It significantly increases the R-factor while, at the same time, providing the necessary vapor barrier.
6) Aluminum paint works well as a barrier for the floor, though Ice and Water Shield (bituthene) probably does too. (The floor must be well-insulated.) Condensation drainage may have to be a floor design consideration.
7) Ample circulation through the stack (using the cfm formulas and other considerations related at this site) is a must.
8) Ample ventilation is necessary, in order to introduce adequate fresh/dry air... especially with softwoods such as pine.
9) Much of the "automatic-gadgetry" (beyond a timer or 2) to control such a kiln is probably overkill.

Any discussion appreciated!

Forum Responses
(Sawing and Drying Forum)
From contributor A:
I learned about gaskets for the lumber stack. Want to make sure there is the right amount of distance between stack and walls, and other areas are sealed so air can't sneak around stack. Also, some people use screens on the top 1/3 of stack to encourage more air through the bottom of the stack.

From contributor B:
I concur about the gadgetry. Anything beyond a timer and thermostat is overkill. You may want a dry and wet bulb thermometer to monitor temps and RH. Maybe a minimum/maximum thermometer to help you set your thermostat. I like the solar kiln concept because of the natural built-in safeguards of not drying too fast.

From contributor C:
My plan is to build a solar kiln that requires no electricity. Here is my two cents worth.

I am drying birch. This birch is air dried for a year before going into the kiln. Kiln is solar with a solar powered gable fan. Low CFM but time is not a factor. I plan to dry about 500 ft per year. After the first year I will have a backlog. I use this wood myself and working full time does not give me much time to do woodworking so therefore I don't need much wood. I will have two whirlybird vent stacks drawing air from the bottom of the kiln. I am almost sure it will work, but I won't know until next fall.

From the original questioner:
I contacted Vaisala about their (very nice) RH and temp sensor/transmitter, for use as a kiln controller. I read where at least one person used it. The cost stopped me. Looks like Krona insulation is only available in 100' x 4' rolls via a manufacturer in Canada. I haven't requested the price yet. Read that one person used it with excellent results. The reason I landed on Krona was because there had been discussion (in archives) about the high-temp sensitivity of "similar" products. Whereas, Krona can handle temps up to 300 degrees without degradation.

Yes, the cfm-thing can require work: air sneaking around, turbulence versus even flow, static pressure, etc. Wondering if anyone's had problems with fans rusting.

FYI: I plan to build a slightly modified version of Kiln #23 & #29, under "Solar Kilns Part 4" in the Knowledge Base.

From Gene Wengert, forum technical advisor:
I agree with (a) through (E). You do not need (f), but it can be helpful to have such readings in case you have some problems.

The best solar kiln design is at

From contributor D:
After studying the material again which is sited by Dr. Gene above, I have a question. In the material developed by Wengert and Meyer, concerning their solar kiln design, a comment is made that "the temperature can reach 150 degrees F". Is it common in the lower latitudes to be able to reach this heat level? In northern latitudes, could a solar unit, with a slight heat boost, do so as well? If so, then the necessary heat to kill powder post beetles and other vermin is met, true?

During the last stages of drying, then, could one deliberately focus on temperature, re-circulating with the vents closed, assuring a sure kill of the potential bugs in a stack?

From Gene Wengert, forum technical advisor:

You can get 150 even in the northern climates in the summertime.

From contributor B:
I regularly got to 150 degrees after I installed the second layer of glazing, especially in the latter stages of drying.

From the original questioner:
Glazing: So, everyone is pretty convinced that the difference between translucent fiberglass-reinforced polyester panels versus something more solar-efficient isn't worth the added cost? However, that second layer is worth it?

Contributor B, what did you use for your two layers? Are you getting the heat gains you expected?

I live in a hail-prone area, and am very concerned about panel damage. Was thinking of covering everything with 1/2" wire mesh spaced about 1" out from panels. Thoughts on this?

Gene, thanks for the 'best design' link. Yeah, I know that's a good one. My problem is that I must keep the height down for aesthetic reasons, and also have footprint width (N-S) limitations. I've already reduced the charge size from 4'w to 3'w x 4'h x 10'.

Hence, I've landed on dryer #29 (with modifications). In that way I can keep the overall height down, and can get two seasonal sun-angles (4 per year)... even though the best angle should probably be geared solely for winter gain. Plan to use black-coated sheet metal over plywood for the collector, placed about 10" behind the glazing. Remaining air flow channels would be a minimum of 12" deep.

I anticipate that the major problem will be with air turbulence at the back of the charge, near the floor along the back wall.

From Gene Wengert, forum technical advisor:
Black painted plywood and black sheet metal will perform the same. You are going overboard on this item.

#29 is okay, but its effectiveness (and the effectiveness of any solar collector) will be the shadow it casts at noon. In this respect, the two angles do not really help; in fact, they increase the footage of the collector and heat losses without increasing the solar input. This kiln was designed to help with low sun angles, but if the outside temperature is under 40 F, you will not get a lot of drying inside the kiln.

Please make sure you have studied the introductory sections (Part I of this publication). The shadow at noon in the spring or summer should be 1 sq ft per 10 BF capacity.

Two layers of glazing are essential. They reduce heat losses considerably and this in turn means more heat and then the heat provides the lower EMCs needed to achieve 6% final MC.

I have studied the Krona insulation site. It does not give (at least I could not find) the R-value of the insulation. Why not? This is really the key to good insulation and helps one compare various products and prices. The large size of the air pockets would indicate to me that it is not as effective (inch for inch, and maybe $ for $) as blue board or many other insulation products.

Krona also talks about radiation reflectance, but inside a wall, this is not an issue. It also talks about convective losses, but inside a wall, this is not an issue. So, can you find an R-value?

Also, the vapor barrier property is helpful, but not essential if you use polyethylene sheeting. In fact, how do you fasten the Krona to the studs so that there are no vapor leaks on the edges?

In short, I am not sure that the extra money would be well spent. Maybe someone can clarify.

From contributor B:
I used regular fiberglass roofing panels on the outside sealed with the pre-formed foam strips that match the "wiggle/waves" of the fiberglass. I used decking screws and small washers with rubber on one side to seal the holes they made to attach the panels. I got the fiberglass panels from Lowe's. I then used battens and put 100% light transmission UV resistant greenhouse film on the underside or inside for my second layer. Yes, I saw immediate results from the second layer, it holds the heat in much better now.