DIY Build Solar Kiln

[Mesquite cutter]

I am going to try my hand at making a solar kiln.  I am going to follow the Viginia Tech design.  I am going to try to just use a vapor barrier.  I am going to skip the insulation for now.   I think with the weather down here in South Texas I may get away with it.   I am more concerned about the humidity than the temperatures.  It is hot down here but very humid.   I will install the exterior panels with decking screws just in case I see that I need insulation.  I can just remove the screws and install the insulation.  

[wkf94025]

Having built and operated two solar kilns for the past couple years, there is no way I would skip insulation, in any climate.  Just my unsolicited dos centavos...

[Ianab]

The solar kiln will still work in your humid climate, because as you warm the outside air via the solar collector, it's R/H will drop. Then the fans blow that thru the stack of wood. It picks up some moisture, then gets vented. This works no matter what the outside temp or humidity are, If the air is warmer and drier, it can remove more moisture from the wood. The collector size and venting control the speed of drying. 

Insulation may not be as important in a warmer climate? 1" of wood does have a R value of about 0.2  Add in an air gap between your interior vapour barrier and outer cladding, and you get another 0.18 R  So it's going to have "some" insulation. But compared to even 1" of polystyrene which is ~0.9 R. 

So that determines how much heat can be kept in the kiln doing useful work, vs how much wafts off into the atmosphere. I'm sure it will "work" as you plan, but it's going to work better / faster with a bit of insulation. 

[doc henderson]

The temp max achieved during the day will be higher with insulation.  It slows the ongoing losses as more heat is picked up.  the drop at night will be slower with insulation.  the change of the outside air to a higher temp, determines the drop in RH.  If the walls get cooler at night, you will have more condensation that needs to be drained or possibly risk rewetting the wood and or turning it back to water vapor when the kiln heats up. the suns energy will then be used to dry the floor before drying the wood.  you could consider using sawdust as insulation.  It must be dry, so you do not have spontaneous combustion.

[Mesquite cutter]

If I install insulation, would I need insulation in the floor also or just the walls?   I think the Virginia Tech model does not call for insulation in the floor, just the walls.  

[Mesquite cutter]

Another question,

I am located in South Texas on the 26th latitude.  Would it be best to build the pitch of the roof at 26 degrees or close to it?  I think 45 degrees is too steep for the area that I am located in.  

[doc henderson]

some make a compromise winter and summer.  some would maximize the angle for the coldest time mid-winter so it can be used year-round.  I think we just pasted the winter solstice.  I would go out at noon and measure the angle of a stick pointing straight at the sun.  if you told me, you would only use it in the summer, I would go lower (more straight up).  If you developed a kiln that you can adjust the angle to the sun and rotate it to keep the glaze perpendicular to the sun throughout the day, you will be a hero to us all.  to keep it simple, in Ks we might just go 45° like the VT model.  Dr. Wengert has told me, "It is only a solar kiln" so they will work well enough and better than open air drying alone no matter what.

[doc henderson]

 Many factors affect how much heat can be obtained from the sunlight. One is the slope of the roof, which in this design is a 45-degree angle to the south. The optimum roof angle is dependent on your location and is typically equal to the latitude of your location. One difficulty in choosing the optimal roof angle is that the optimal angle for solar collection changes with the season since the angle of the sun changes. If you are planning on operating your kiln during the winter months, you can improve your collector's performance by increasing the roof angle another 10 degrees. For example, Blacksburg, Virginia, has latitude of 37 degrees; therefore, the optimum angle for winter operation would be 47 degrees. We compromised with a 45-degree angle to make construction easier.

the above is taken form the article on the DIY solar kiln, I assume you have.  so maybe 35°

[Ianab]

General rule is make the collector the same as your latitude. So maybe ~30° would be better. I also assume you will get some drying action all year round. Maybe a bit slower in Winter, but not the ~6 month Winter cycles the more "polar" folks get. Some insulation should improve the kilns performance no matter what, even something basic. 
 

[teakwood]

I insulate my kiln that i'm building right now and here we are in the freaking tropics. I think the insulation will give me at least 4-5hours more drying time in the night.

https://forestryforum.com/board/index.php?topic=121021.0

[Mesquite cutter]

Thank you all for your replies.  It really helps in making my decision.   I will insulate the walls and just place a vapor barrier in the floor.   I will be using it throughout the year.  Winter only last about two weeks here.  We are in January and we still need to mow our lawns around here.  For example, it will be a high of 79 degrees today. 

[doc henderson]

if you point it at the sun now, it will work better (best for winter) in the winter, and still a percent less than optimum in the summer (for summer) but better overall and still better (in the summer) than in the winter.

[Sod saw]

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As in most places, hot air rises as cooler air sinks.  

In your kiln, the fans will help overcome this tendency for the air to layer.  However, the cooler air will still attempt to stay near the bottom of the kiln chamber with the warmer air attempting to remain near the top.

If you don't insulate the floor, you will be encouraging the lower air to be kept cooler by the cooler floor location.  If you do insulate the floor, the mass of that cool outdoor air attempting to cool the inside air will be somewhat blocked (insulated) from influencing the lower kiln air.

Why is this important?  With a full kiln load, my wish is for all of the wood to be the same dryness (moisture) at the same time so the full kiln can be all unloaded at the same time.  If the lower portion of the load is cooler, it will probably take longer to dry (maybe still too wet) and perhaps the top of the load will be dryer (maybe too dry).  It may take longer for the load to be ready.

Can you just add more fans to stir up more air?  Yea maybe, but how about the cost of replacing extra fans and running them? How many years will you intend to use this kiln?  if you amortize the cost of some floor insulation and consider the added load or two per year in the kiln, will you be further ahead by adding insulation or not?

I wish that I had spent the money to spray foam my insulation instead of using foam boards as those boards have gaps between sections for air and heat loss, even though I did spray foam these board joints.  I can walk around and feel the difference with my hand on the outside of the kiln walls .    Yes on hot sunny afternoons in the summer, when the difference between inside and outdoors is 50 or 60 degrees F.

Just my thoughts and opinion, but I do agree with teakwood as to extending the length of the drying day.

have fun with your build


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