Solar kiln glazing

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Best material choices for a double layer of glaze. April 9, 2003

I finally decided to build a solar kiln. Instructions in WOODWEB's Knowledge Base call for two layers of clear fiberglass spaced 2" apart. I went to Lowe's yesterday and decided to use polycarbonate sheets instead. They have it in clear and smoke. Would it be better to use the smoke tinted sheets for the inner layer of glazing? It's pretty dark and one would think it would attract more heat than the clear. The price is the same.

Forum Responses
(From WOODWEB's Sawing and Drying Forum)
From contributor K:
Using the 'smoke glaze' on the inside sheet would likely cause an interruption in the rays and cause the area between the panels to heat. That would, in turn, cause a heat loss through your outside layer. If a person wanted to collect in the way you're describing, they would probably put another panel of heat-conducting metal (like aluminum) on the inside of the double glaze. The metal would be painted black on the sun side and have gaps top and bottom for air flow. It seems that if this were only on the bottom 1/3 of the structure that there would still be plenty of daylight in the kiln. Any extra heat generated from this would then be distributed in the lower part of the space. Don't know if it's been done in a kiln yet.

From contributor J:
Right now, I just have one layer on my roof with the clear fiberglass you can get from Lowe's. I have enough to put the second layer on, but still haven't figured out what would be the best way to get a seal between the two layers. It seems to me the purpose of the two layers would be to have this space like a double pane house window with no venting between the two layers. My roof is made like a regular roof with rafters and purloins, and I used the foam wavy strips to make a good seal. I have thought about finding some type of clear fiberglass that is flat and just attaching that directly to the bottom of the rafters. This would result in an airspace of 3 3/4" since I used 2 X 4's for my rafters, so I don't know if I would benefit very much. Using some type of flat fiberglass, maybe from a greenhouse supplier, in theory would seal up very tight since you aren't worrying about trying to seal the "waves." I also tried attaching a piece of black painted metal to the bottom of the rafters to act like a collector, but this shadows the light from the fan baffle and stack baffle. On the present charge, I removed it and am trying this charge without it. I am curious to hear how others have put double glazing on the roof using the wavy fiberglass panels.

From contributor K:
You're right. You don't want ventilation between the double glaze. If metal is used it goes to the inside (you know... inside the building). Sorry - that wasn't very clear before. On the second piece of glaze... it seems like a person could fasten some 1x2 against the flat side of two parallel rafters. Now, if before it was fastened, spacers of 1/8" were placed between it and the purling, there would be a 1/8" slot left after the spacers were removed. Would that be enough clearance to slip a flat sheet in place? With the details of sealing the ends and caulking, of course. You have a good point about 3 3/4" being a lot of space. Something to do with what the sun's rays are capable of. When they're so far apart, they are sort of out there by themselves as opposed to being a unit, too. It would be well more than twice as good to do the glazing double, though.

You want to maximize the amount of solar energy getting inside the kiln, not inside the glazing, so use clear.

The spacing is not critical between the layers. The space also does not need to be sealed...we just do not want air flow through this insulating layer of air. A little air flow is okay as that will carry away any excess moisture.

Gene Wengert, forum technical advisor

From contributor K:
Gene, thinking in terms of heat gain makes sense, but wouldn't heat loss also be a consideration? In other words, coming very close to achieving a dead air space between your panels. The other thing about air volume between the panels must make a difference by reason that this air will be heated and make an "average temperature" buffer to the inside/outside TD. If panels A are spaced at 1" and panels B are spaced 2" the volume of the B panels would be twice that of A's. At some point the difference would be significant. There is also a question of what the solar rays will do in a given distance for the purposes of gain. What are your thoughts on this? Of course, if it interferes with getting the two panels up (or better, three) let's get it done, right?

The energy used to heat air is insignificant compared to heating water and evaporation.

A perfect seal would be the best indeed, but the benefit is so small that it is not worth the cost for a solar dryer.

Gene Wengert, forum technical advisor

From contributor J:
Thinking out loud here... If I went to a greenhouse supplier and got some clear sheet plastic and stapled it underneath my roof, that should help substantially, correct? It wouldn't be a perfect sealed airspace, but it would be a second layer of glazing. But, most greenhouse material is UV resistant, so then you would be defeating the purpose of the second layer, right? What if I just used regular 6 mil clear plastic that wasn't UV resistant? It would probably only last one charge, and disintegrate like my first roof, but it would make a second layer. I can't visualize how one could use the wavy corrugated fiberglass as the interior glazing and get a good seal between the roof and walls if the roof is hinged.

I suggest you try building a rabbit and butting the wavy fiberglass up against the rabbit. Next time you see a "tin" building, look at how the walls butt up against the rabbit on the concrete slab. Do the same with your boards and fiberglass on the roof's perimeter.

You can buy corrugated wood to help seal corrugated sheeting.

The idea of using an inner layer of UV resistant plastic is perfect. The UV resistance does not interfere with transmission of solar into the kiln.

Non-UV plastic will last 6 months at best before it gets brittle and breaks.

Gene Wengert, forum technical advisor

From contributor J:
I must have crossed my wires somewhere in my research about using UV resistant material on my kiln. I can get clear flat UV resistant greenhouse plastic real easy, but never did. I thought UV for some reason was important in the heating process and it was needed. It would be easy to put that on the interior of my roof and have the second layer of glazing. Is UV important or do I need to go back to physics class?

You can use UV stabilized plastic for both layers. No problem.

Gene Wengert, forum technical advisor

Doesn't UV cause degrade, as in breaking down the lignum in wood?

From contributor J:
The fan baffle and charge baffle intercept the rays before it affects the lumber being dried. If any damage is done, it is to the kiln itself. The lumber charge in my kiln is actually shaded from any sunlight.

Just to follow up, I went to my local greenhouse supply store and got a piece of greenhouse UV resistant 6 mil plastic with 100% light transmission for $5. I'm going to put that on the interior part of my roof and see how that works for heat retention. The salesman said it should last 4-5 years.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
UV resistant does not mean UV blocker. The material itself is resitant to damage caused by UV.

Comment from contributor B:
I am new to drying wood, and am investigating solar kiln drying to produce wood for my hobby. I do maintenance, repair, and construction at a large greenhouse in Michigan. There are several kinds of material that are great for letting sunlight in and are UV stabilized. Polycarbonate sheets come in single, double, and triple layers, with air spaces in between. These can be up to 16 mm thick. There are also aluminum framing and glazing materials that can be purchased to seal these panels up nearly 100%. There is also an acrylic material that is single, double, and triple thickness. Also with aluminum glazing material to seal them up. The polycarbonate material is cheaper, but doesn't transmit light as well.

We have the acrylic material on most of the greenhouses where I work, and it has a lifespan of at least 10 years. The polycarbonate doesn't last quite as long, as far as light transmission goes. The acrylic and polycarbonate can be walked on during construction. The polycarbonate can be hit with a hammer very hard, and will sustain no damage. The acrylic isn't as tough as the polycarbonate material. As they get older, they will both get brittle. The polycarbonate will be quite tough for a long time. The acrylic material can't be point fastened, while the polycarbonate material can be.

6 mil plastic can be purchased from greenhouse suppliers, as most of you have found. We use this for up to four years, before it gets too foggy and doesn't transmit light well enough for plant growth. Using a small squirrel cage fan to inflate the space between the two sheets of poly film will keep them taught, which adds to the life of the film. It also helps to cut down on condensation on the inside of the structure, since the outer and inner layer don't come into contact with each other.