BUILDING MY SOLAR Powered DRY KILN

INTRODUCTION
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Being able to kiln dry your OWN lumber is a major benefit to a woodworker. This is especially true since many custom projects require species not available at box stores. Also, owning a kiln allows the furniture builder to use locally sourced lumber from logs which, many times, are left to rot where they fall. Owning your own kiln is a necessity if you are fortunate enough to own your own sawmill. The potential projects can then become limitless with this powerful combination.
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I came across the best Solar Dry Kiln design built by a local former military fellow, a great friend of mine, who was over the computer room in Huntsville, Alabama during the NASA Apollo Space program. His design is hands down more efficient than many other small solar dry kilns you'll see available. It can get as hot as 200 degrees Fahrenheit inside the kiln during the summer months here in the southern United States. The first photo shows the front of the kiln and five large doors. The center three are removable, and the two end doors are hinged. It's 18' long and 5' wide. If you build one, consider the length to accommodate whatever size your mill can handle. Just add (or subtract)  the removable center doors as needed.

Solar Dry Kiln

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The system is basically a collector panel plenum (the area between the rafters) which heats up with the sun’s rays. The air is circulated through the lumber stacks by a couple of metal blade fans. Additionally, a dehumidifier is added inside to collect the moisture drawn from the wood. There is no other heat source but the sun, and temperatures can reach in excess of 180 degrees Fahrenheit. It can accommodate 16' long lumber easily. I also added a trailer hitch attachment in order to move it, although it's now in its permanent home.
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Solar Dry Kiln rear view

A Load Of Walnut Inside the Kiln

 
In the photo above, you can see the dehumidifier unit on top of the stack, as well as two metal blade fans that blow down from above. The main idea is the sun heats the black corrugated metal sheet that is attached to the bottom of the rafters facing up to the sun. The clear roof panels attached to the top then create a plenum between the rafters (sort of like a tunnel). If you’ve ever grabbed a black handrail that's been exposed to the sun, you know how hot it can get. That’s why the metal is painted black and facing up to the sun. It gets REALLY hot. The upper plastic roof panels on top trap the heat which rises to the mini attic and recirculates down by the fans blowing down. The air flows through the lumber and reenters at the rear of the roof (the lowest roof point.) Take a look at the drawing below:
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Here's a video of me describing the kiln from a few years ago:

https://www.youtube.com/watch?v=wTnvkeDsp7c&t=6s

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​FOUNDATION
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I used a wood foundation on blocks. You can do this, or use a reinforced concrete slab. I began by cutting lumber I needed on my Norwood Lumbermate 2000.  The stock needs to be air dryed so that it will be stable when applied.
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Milled Lumber Used on the Dry Kiln

​I had a trailer hitch attachment that came with my mill and an old axle laying around, and decided to use them. I wanted to be able to move the kiln to a different location later. The original hitch simply attaches with a removable pin. (If you build a kiln that remains fixed, it's unnecessary). The custom piece was made to attach the hitch to the kiln. My buddy Wayne welded it together for me. You’ll get the idea of where it goes in a bit:
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Custom Trailer Hitch Metal Attachment Piece

Old Trailer Axle

​Now time for the fun part. I began by laying out some blocks I had, and did some heavy calculatin'. I wanted the kiln to be 18' long and 5' deep, and so I leveled them up to fit that. Then I placed some of the supporting foundation lumber on them: 
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It Begins!

​You can see the notches where they were laminated (lapped) for strength. The 2 long sides were made of 2 layers of 2"x6" lumber lapped in the middle. The two end and center pieces were 6"x6". I bolted them together at each joint. The blocks used weren't uniform. I just needed something to support the base. Also I happened to catch the corrugated metal I found at the dump (see to the right laying on the ground) that I would later use under the rafters after I'd painted them black. Here’s a little more progress:
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More Foundation Beams in Place

Attached Axle

​And finally, I had the base beam system completed. I sprayed on some black paint for a little protection. I also bolted on the custom piece the the trailer hitch would sit under. I did a bit of notching to make it and the inserted hitch flush with the top. I bolted it through both beams so the notching didn't weaken them. 
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The Completed Foundation Base

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​FRAMING THE SOLAR DRY KILN

Next I added the floor joists. They consisted of 2″x4″s spaced 16″ center to center. Notice in photo below that the added floor joists are set back 1” from the front edge. This allows the front doors to fit. I’ll cover that a bit later on. I added some blocks where any boards were joined together for strength. Then I nailed the completed joists down into the beams all around with 12 penny nails. I HATE 16 pound nails. We used to call those sucs-teens. :)
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2"x4" at 16" on center Floor Joists

​Next I installed a layer of 15# roofing felt, and added 1" thick floor boards. You can see a small hinged access door over the hitch I made for the hitch pin.
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1" Decking (Floor) Boards over 15# Felt

​Then I began framing the walls. To first determine the height I needed for the front wall (the back wall is 4' high) I drew a 45 degree angle on the floor and popped a chalk line. This gave me a full sized template I could use (I'm basically drawing the section I showed earlier on the deck full size).

Quick Sketch How To Determine Front Wall Height

​The 45 degree angle is simply a good one for my area. It could change further north to adjust for the sun rays. But now you know the height of the front wall. WRITE IT DOWN! You might forget (I do that a LOT nowadays). So then I laid a 2"x4" on the floor drawing and cut a pattern rafter, remembering to have the flat area at the top and bottom to fit the layout. Next, I stood up the rear wall seen below, and next the two end walls the same 4' height. Progress!
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Rear Wall in Place

​The front beam over the doors was made using an engineered design I found on the internet. The site has since disappeared, but after a number of years, the beam has not sagged. Of course you can buy and engineered beam sized for the length if you'd like. But mine didn't cost me anything except time and nails.

Here’s a couple of pictures of the beam being built. You can build one like it by doing EXACTLY as it's shown in the photos if you used an 18' length. Good luck if you went with a different size. :)
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It’s built out of 2″x4″s. The top and bottom are full length boards, and connected with blocks at a SPECIFIC spacing…and like I said, I don’t remember from the original design the exact spacing. (It looks like there is a block in the center to me, and then another spaced between the center block and the ends). I filled it with batt insulation and then applied some leftover 1/2" plywood I had to both sides. The nailing pattern on each side was intense, and REQUIRED so many nails per inch. You can see the large number of nails in the bottom photo.

​OK, now pay attention to this. I had forgotten until now how I'd framed the front wall offset. I just went out and took a look, and it occured to me what I'd done. If you'll zoom in on the photo below, you'll notice a 1" short board cut to width that is nailed on the two front ends of the foundation. That is there for the door channel. The front wall studs sit flush over them. Take a good look, and you'll see what I did. Do the same.

​After cutting the front corners to the correct height (that was determined by our rafter layout sketch on the floor)  I man-handled the now HEAVY beam into place by myself. That was a mistake! Get help to do this! It was a harrowing experience, but I persevered. Here’s a picture of the walls and beam in place:
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Completed Walls (Less the Gable ends Added Later)

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​Now you can see the completed wall framing and the front corner framing. I beefed the corners up that support the roof, since almost all the weight is on them. I added a top plate and 2"x4"s at 16" centers over the beam at this point which isn't shown. Next I plumbed the walls, and braced them. Then I sat a rafter into place. It fit great! I guess the goats liked it too.
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The First Rafter In Place

​It looks as though I got a bit excited and installed a bit of exterior lap siding on the rear wall. I don't remember why! Anywho, it's necessary to apply a layer of 15# felt to the wall to act as a vapor barrier first, and then add the siding. Continuing on, I added a few more rafters, and then completed them all.
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A Few More Rafters in Place

I went ahead and filled in the two gable ends with 2"x4"s in line with the ones below, and then added felt and siding to complete the exterior end walls. I used 1"x4" trim boards on the corners first, and butt the siding to them. 
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End wall Framing

​I added the electrical next. I placed an combination outlet/switch near the bottom for the dehumidifier plug and single switch for the two fans, and also a dual outlet at the top center over the beam for the 2 fans.Then I placed another outlet box below the first one facing out to plug the kiln into an outdoor extension cord. You can see the two boxes on the end and the one on top of the beam in the middle. And here’s a picture of the exterior outlet box I use to hold the plug for the kiln:
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End Wall Outlet Box 

​Next I insulated the walls and began adding the interior 1/2"x8" boards I milled. Be sure to put the insulation kraft paper facing IN on the heated area. Also, note a flat 1"x4" was nailed to the top of the end gable studs to have a nailer for the interior metal ceiling.You can see it sticking out on the far end of the second photo. We call that "dead wood" when we were framing houses. 
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Insulation is added and Interior Siding

​I put felt on the front. I also added a 2"x4" set back 1" from the front onto the bottom of the beam which is shown below. This is the top door channel area:
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Felt Applied To Front

Continuing on ... The picture below is from the inside looking up and shows the 2″x4″s in place over the 2″x4″ rafters. You can see the trim board (dead wood) mentioned earlier over the top of the end walls used as a nailer for the interior metal ceiling ends. The clear panels from Home Depot were then screwed onto the top of the 2"x4" runners. Allow about a 3" overhang all around. All the siding should be cut to butt up to the panels for added support to prevent sag and air leakage. 
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Roof Panels Applied

I then added some blocking for the fans to rest inside. You'll need to make SURE you use fans with metal blades. Plastic blades will melt with the heat. Choose a model that will fit into your space, and block to fit. You also need to make sure you make it removable for future replacement when needed. If fact, it might be a great idea to buy a couple of extra ones and store away for that purpose so they will fit well later too. I've had to replace a couple over the last ten years.
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Fans In Place

​One day I took a load of trash to the dump. It just happened that a fellow was throwing out some galvanized old metal. It was perfect for me to use on the next step of the build, and so I loaded it up. When I got back home, I laid it out on the ground, and caulked all the holes in it. Then I spray painted it black on one side:
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Painting the metal black

​This was then screwed onto the bottom of the rafters with the black painted side facing up. THIS NEXT STEP IS IMPORTANT! Leave about a 6″ gap at the top and the bottom. Look back at the section. This allows the air to enter and exit from the plenum that is created inside the rafter cavities. I then added some ceiling boards between the fans and the ends to create a mini attic. This traps air and causes it to collect more heat. Then I added some boards between the fans and to each end to create a mini attic. Next I insulated it with the paper back facing DOWN. It was NOT fun! Here’s a picture showing the added ceiling boards:
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Mini Attic Installed

We're almost done!! Thanks for sticking with me, and I sure hope that this means you're thinking of building one. I would encourage you to if you own a mill. It is the most beneficial thing you can do to be a great woodworker.

The next thing I did was build the front doors. The two end doors are hinged, and the 3 center ones are simply lifted and removed. They sit in a channel I made on the bottom and top. It is about 1 1/2″ deep on the top and 1″ on the bottom. This allows me to lift the doors into the upper channel, and let them down into the lower channel. They remain about 1/2″ into the upper channel keeping them in place. 

Next I added a long 1" thick board cut 1" less in width than the two short end boards we nailed earlier to the front foundation. It goes the entire length of the front between the two short boards, nailed to the front foundation, and kept flush with the bottom of the two existing end boards. This is the beginning of the lower channel.

Then I added 1"x4" trim boards around the door area. The two end 1"x4"s are flush with the studs. The top 1"x4" is offset down 1 1/2" below the beam to create a 1 1/2" deep channel to hold the doors along the top. (Remember the 2"x4" we nailed onto the bottom of the beam earlier? That's now the back of the top door channel.) The bottom 1"x4" is placed with the top 1" above the long board we just put along the bottom of the front foundation.

I began making the doors by building each of the five frames out of 1″x4″s using biscuits and glue. Make the middle ones all the same size. To determine the height of the middle frames, measure between the channel areas we created earlier along the top and bottom and subtract 5/8". The hinged door frame height is 1" less than the distance between the OUTSIDE upper and lower trim boards just applied. They should clear the trim about 1/2" on the top and bottom when they are put on the hinges. Just make sure when you install them that they are inline with the channel when shut.

Now measure between the two front trim boards all the way across the front. It should be somewhere around 16' or a bit less. Then subtract 2 1/2" from the total length. Then divide by five. That gives you the width of all the frames. The 2 1/2" was subtracted in order for the frames to have a bit of a gap between them to fit in the channels easily. Since the siding will overlap each frame, I didn't worry about air leaks.

Next, I sat them into place using hinges for the ends, and letting the center ones rest in the channel spaced correctly into position. They have about 1/2″ clearance between and fit loosely. I then applied the siding on each door using the method below. I offset the siding, one door to the next to have it overlap onto the next door which helps to hold them in position. Below are a couple of photos to get the idea. I ran a string along the length to keep them straight also:

Front Door Frames Set Into Position

By placing the frames into the channels and THEN adding the siding, you can keep the pieces straight by using the string from end to end on each row. Let me try to explain about the siding length for each door.

The end door pieces are cut 1 1/2" less than the width of the end frames. It is applied flush to the hinge sides, and screwed onto the opposite end allowing 1 1/2" of the frame to be exposed. This is so the next door's siding can sit on top of the end door frames. Is that clear as MUD?? Well, it is totally necessary for the doors to stay in alignment and also seals out air.

OK....listen up. The siding for the two doors next to the two hinged doors is cut the full width of the frame! Apply it 1 1/2" outside the frame in order to lap the hinged doors on the ends. You'll have the same exposed 1 1/2" on the opposite side. Lastly, the center door siding is cut 3" longer than the width of the frame and applied so that you'll have it sticking out 1 1/2" on both sides. When they're all set in place, you should have a nice front siding job. You'll probable figure this out when you do the bottom siding first row. Just take your time and you'll be fine.

I just went and took some pictures of my kiln that may help you a lot. Here ya go:

Front Door Details

My poor kiln has taken a lot of abuse through the years, but still looks pretty good, and still works great! But time to move on. (I'm a bit winded after hustling to take the pictures. remember I'm gettin' up there.) OK ... here we go....

So after the front siding is on the doors, I laid each door on my sawhorses with the siding down and cut some 1" styrofoam to fit:
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Installing the Styrofoam into the Doors

Then I applied the same 1/2″x8″ material I used for the inside walls, being careful to make it the correct width for the doors to fit. I guess I don't have a picture of that. Just make sure you cut it to fit to clear all the inside obstacles....floor, etc. Maybe just mark where the siding goes BEFORE you take the doors off with a sharpie. You'll need 1/2" clearance.

The final step was to install a heavy cloth tarp which is used to lay over the lumber stacks when drying. It diverts the air back into the rear upper plenum 6″ area I made earlier when I installed the metal. I attached it (with much effort, it was not fun) along the top at the end of the metal ceiling pieces. Here’s a shot of the tarp I used. Get a heavy duty one and cut it to fit. It drapes over the finished stack maybe a foot or so. This directs the air from the rear of the lumber stack back up to the top rear of the kiln. 

Rear Tarp That Drapes over the top of the Lumber

​Here's a photo of it installed. I used a 1"x4" to fasten it to the metal and hold it in place. I probably just marked where the board went (4" up from the end of the metal) and secured the tarp to the metal first. Then I put the 1"x4" on top of it to hold it in place. REMEMBER to make sure you are clear of the rear 6" void above. 

The tarp in Place

The last few steps are to finish the front corner trim and siding. Remember to butt the top piece up to the panels for additional support. Caulk all the joints around the kiln with acrylic latex caulk, and paint with an exterior paint for protection. Don't put this step off for long since it can harm your siding.

Hey, that's all I got on this project. It was fun going down memory lane with this. If you have a bandsaw mill, let me strongly encourage you to build one. It's not very hard to do, and if you have enough mechanical skills to use a mill, you are definitely handy enough to build one. So get busy, and THANK YOU FOR READING.