Question
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.
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.
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.
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.
The best solar kiln design is at
http://www.woodweb.com/knowledge_base/Processing_Trees_to_Lumber.html
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?
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.
#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.
