Why Insulate a Solar Kiln?

A long discussion delving into the mechanics of how a solar kiln creates proper conditions for drying wood. January 28, 2014

Does it make sense to insulate a solar kiln when the bulk of your heat loss is through the collector, the thinnest wall? I can see it if you close a lid on it a night and tuck it all in for bed, but see the insulation as a waste of time and money otherwise. Any thoughts?

Forum Responses
(Sawing and Drying Forum)
From contributor T:
So why do you have it in your house? To help protect the inside temps from the outside temps. Once a solar kiln has gained its max temp and the wood has absorbed its max as the exterior temps drop, and that's usually fast after the sun drops, the wood serves as a thermal mass of heat/energy and with insulated walls/floor it can retain that higher temp longer during the end of its cycle.

From contributor C:
Well, yeah, but since the collector is the weakest link with no R value (like R.5) and heat rises and all... The insulated walls are no more than a thermal insulated funnel as the heat flits out the top.

My lo-tech VA Tech plywood box is uninsulated and I have yet to feel it would really benefit much from it. It is really efficient in drying. I double/triple film the top and of course the load is a thermal mass. But without covering the collector in the cooler hours, I do not think walls insulated or not you will maintain the heat. Seems like an unnecessary expense.

I have thought it would be beneficial to have a cover similar to those made for hot tubs. Do any of you out there cover the collector at night?

From Professor Gene Wengert, forum technical advisor:

If you have a two layer collector, you will lose about 25% of the energy that comes into the kiln through the walls and floor without insulation. We need this energy (heat) in order to evaporate moisture and to lower the RH so we get low final MCs. Performance will be greatly affected.

From contributor D:
On the next sunny day go out and feel the shaded side of your kiln wall from the outside. If you feel heat, you are losing heat through your wall.

From the original questioner:
I guess at my latitude, 19*, I am as proficient as I need be. I average 115* (just before air hits the fan) most days, and much as 140* on a particularly hot one. I can still be at 90* at 10pm. I think the material would still be 100*. Just 1/2" ply painted black. I even have to close down part of the collector with smaller loads. Is there anyone else here with a solar kiln in the lower latitude?

From contributor I:
Heat does not rise, hot air will rise.

From the original questioner:
Very good point. So, what are we dealing with in the kiln? A radiant heat from the solar mass? Or does the solar mass heat the air around it which goes out the top? Still to me, since the collector really does not slow heat from leaving the kiln, the kiln is like a box without a lid. I think my next version of the solar kiln will insulate and have the hot tub cover for night. I have the kiln right outside the shop I work at most days and observing and tweaking is fun and easy. Still would be nice to see some automation of sorts - vent, etc.

From Professor Gene Wengert, forum technical advisor:
A two layer collector does indeed stop a lot of heat loss, both radiant if using a low emissivity product (like glass) and convection with the space between the two layers slowing conduction quite well. So, it is not even close to an open box.

The British have already published a vent control system that is computerized. Too expensive and little benefit.

Do not cover the collector at night, as we need it to cool so we get the high RH to relieve stress. Maybe near the end of drying you could do that, but the RH will go high and that will stop you from getting low final MCs. That is why we do not run fans at night.

From the original questioner:
Hmm, space between the sheets, eh? I am thinking the film is expensive and short lived. I will have to think more of the MKII version to be released.

Not to be thick, but we are comparing two layers of film (with air space now) to insulated walls? For my non-volume based biz, uninsulated is fine.

I have been running a 64 pint dehumidifier from 9pm until 9am when I bottom out with the solar energy route. The dehumidifier helps equalize at the end. It works great. I am not sure if you could use the home dehumidifier for drying from wet, but the last bit doesn't seem to bother the function.

From Professor Gene Wengert, forum technical advisor:
For a cover, you can get "clear" (it is not perfectly clear) corrugated fiberglass. Works great, and then put a second layer of that or a film that is UV stabilized inside. Or you can use two layers of UV stabilized plastic film. Or you can use window glass (but avoid glass that appears greenish on the edge). Or you can use the cover that is two layers with pockets built in and is UV-stabilized.

That advantage of two is that the unit will get hotter and go faster. So you would not need a DH unit and need to use that expensive electricity.

From the original questioner:
It's amazing really, that LG dehumidifier set at 35% rh at night for 12 hours nightly doesn't come on as much as I thought. The wood kind of soaks up the ambient rh I think. The dehumidifier only costs me less than about 7 cents a bd ft I figure. I air dry so I really only kiln from 13~15% AD to 8%KD.

That's interesting - in the other thread that you said there is no stated guide for kiln dried MC for hardwood. In Maine it was 6~8% typical, and here I use 8~10% as a personal guide. Some guys only kiln to 10~12% because they cannot keep it down because of no stable storage.

From contributor T:
This is my theory on the kilning MC. I'd rather take it lower in the kiln and raise back up some in storage, which is usually more of the outer shell effected than the inner of the wood on short terms, than not bring it down far enough internally and have the shell lose MC, causing more critical issues with the wood.

From Professor Gene Wengert, forum technical advisor:
If anyone is in PA, I will be talking about practical aspects of final MC (including correct values, how to measure and how to achieve) at the Keystone Kiln Drying Assn meeting in State College on October 24.

If anyone is in or near London, KY, I will be giving the same presentation topic at the Ohio Valley Kiln Drying Assn. on October 1.

A 12 word summary of part of my talk "The correct final moisture content is the value that the customer wants."

A three word summary of another part is "equalize, equalize, equalize."

From contributor A:
You won't reach as high of temps without insulation if you want to kill bugs or set sap. Our winters here would also kill my kiln without insulation. You see, I use double glaze for collection too to hold in heat, along with insulated walls/floor and if the sky is perfectly clear I will break 160-170 whether it's 100 degrees outside or 50.

From the original questioner:
Where are you reading your temps? That's the highest I have heard.

I'm sticking with the uninsulated kiln. Where it sets it dries faster than I feel comfortable some days. I shoot for quality over quantity most any day. I have not heard of anyone successfully sterilizing with solar kilning yet so have not aimed for it. I can get a pack of lumber fumigated for $35. Kills the termites. Any PPB holes are defect, reduce grade, and are cut off or not charged for. I usually begin my sterilization program as I sticker the wet lumber by spraying. Keeps 99% of the beetles out until the wood is dry enough where they do not care so much about it. I do not leave stacks of air dried around. I air dry, kiln, then store. I live in bug country but have very little in my stocks.

From contributor A:
I read where the air comes out of the wood stack mostly. It only reaches that high when the wood is almost dry of course, but it has no problems when empty. I just took a load out 3 hours after the sun quit hitting it and it was still in the mid 80s. As for bugs, it kills them great. Other than the occasional found in my stack, we put a bamboo bar that my buddy made in there after finding bugs eating the bamboo like crazy. Next day there were 25-30 little dead bugs laying in a line around the bar. I love the power of the sun.

A kiln with no insulation can work just fine really, but if the time comes when you need more drying capabilities, I believe insulating would be more cost effective than building a second one.

From Professor Gene Wengert, forum technical advisor:
In my experience, a kiln with dry lumber and at about noon, the internal temperature can reach 40 F hotter than outside. If the fans are off, the heat at the top of the roof can reach 200 F.

From the original questioner:
Has anyone implemented solar hot water for use in the end cycles? Sometimes I just think if mine was on a slab or had some 50 gallons of water to heat during the day as a solar mass I could really extend the day cycle without spending any money on the dehumidifier.

From Professor Gene Wengert, forum technical advisor:
A certain amount of solar heat comes into the kiln on a given day. You can use that heat when it comes in or you can use part of the heat and store the rest for later. After 24 hours, however, you have the same heat in both cases and so approximately the same drying. (Kiln efficiency and storage efficiency will make a difference; more efficient is also more expensive.)

When you begin to look at adding energy (solar hot water, dehumidifier, etc.), you will quickly see that the kiln will be much more efficient and energy will be used more efficiently if the chamber is fully insulated and the collector is separate. Again, that means more cost for the building and hardware.