Building a Solar Kiln

Building a Kiln As a woodworker in a basement shop I always prefer working with 8 foot boards and sometimes 10 foot boards, but anything longer is difficult to handle in such a small space. I keep this length limitation in mind when bucking and milling logs. Ultimately this directly impacted my choice of kiln size. The construction of my solar kiln began with plans from Virginia Tech for their 750 to 1000 bd-ft kiln . The original plans call for an overall length of just over 13 feet to facilitate drying 12 foot boards. I resized the kiln to an overall length of 12 feet to allow drying of 10 foot boards. This also made interior and exterior sheathing a little less wasteful. Resizing the kiln results in a capacity of about 500 bd-ft for 8 foot boards and 650 bd-ft for 10 foot boards. Solar collectors of any kind are generally tilted at an angle equivalent to your latitude plus 10 degrees if you want to maximize solar capture in the winter. This kiln resides in Lynchburg, Virginia with a Latitude of about 37 degrees which is pretty much the same as Blacksburg, Virginia where the Virginia Tech kiln is located. Adding 10 degrees puts at 47 degrees, but who wants to make those cuts? To make life simple, the kiln roof is tilted to 45 degrees. This kiln is, for the most part, the Virginia Tech solar kiln with a few very minor modifications. Construction required about a year of my spare time, but that included six months or more when no work was done at all. Realistically, this kiln could probably be built in a few weeks of full time labor. I constructed the entire kiln by myself, so if you have more time than resources, it can be done as a one man job. Whenever I looked online at kilns others had built, I always came away wanting more details and more pictures. With that in mind, every attempt was made to take plenty pictures during the construction process. Take a look through the pictures and discussion in each section of the construction process. Hopefully, these pictures, combined with the Virginia Tech plans will make your kiln construction, an easy project. Base The solar kiln base (foundation and floor joists) is pretty straightforward construction. The lot where I built the kiln was a gravel parking space many years ago so the ground is mostly a gravel and dirt mix. This combined with the fact that the kiln may need to be moved at some point (hopefully not) made the use of a skid foundation the obvious choice. The foundation is made up of two treated 6x6s, 12 feet long. I chamfered all four of the bottom edges and drilled large holes in each end. The chamfers will hopefully make dragging the kiln easier and the holes may be used as attachment points. Once the skids were positioned and leveled, the floor framework was built with treated 2x8s. The skids were used as a construction platform, but the floor joists were not nailed to the skids. The Virginia Tech plans call for double rim joists, but since all of the joists rest directly on the skids and none are "hung" from the rim joist, I figured a single rim joist would be fine. Plus it saved me a few dollars. With the floor framework complete it was time to move on to the flooring. The first plywood to be nailed down will become the bottom of the floor framework. Here I used three sheets of 1/2" treated plywood. Remember to save the offcuts, they'll come in handy down the road. Once the plywood is installed it was time to flip the entire floor assembly. Although I muscled this monster over by myself, I would strongly suggest getting a friend to help flip it and reposition it on the skids. The flipped flooring framework (say that three times fast) was repositioned on the skids and then nailed to the skids. I nailed directly through the plywood into the skids and toe-nailed the joists to the skids as well. Insulation was installed between all of the flooring joists. Defense in depth is the strategy for keeping moisture out of the insulation. The craft paper was covered by black plastic vapor barrier stapled to the rim joists. Three more sheets of treated plywood were cut to length and installed over the vapor barrier.

 

Framing Framing for the solar kiln began with a truckload of 2x4s of various lengths as called out in the VT plans. The first wall I built was the front or south wall. Nothing fancy, just normal framing 16" on center. One minor change I made from the VT plans was to utilize a three stud corner instead of the specified four stud corner. The three stud corner allows just a little bit more insulation in the corner to help decrease some of the heat loss through the studs. North wall framing began by constructing the built up beam that will serve as the door header. Two 2x8s, some of the 1/2" plywood offcuts, and about a billion nails completed the beam. The north wall was framed while laying flat on the kiln floor. Since this was a one man operation, I added some additional, temporary studs to the outer edges for the lift. It was easiest to use the south wall as stop and use the truck to slowly raise the wall. Once it was vertical two support studs were nailed to each side. These studs allowed me to slowly "walk" the wall back one side at a time, little by little without the fear that it would tumble down north or south. I would just lift up the support on the north side, drag one end of the wall a foot or so towards the north and let the south support drag the ground. Go to the other end and do the same thing. Repeat, repeat, repeat...... When the wall was flush with the north edge of the floor, it was nailed in place and a temporarily attached to the north wall. No special framing was done on the south wall to help with insulation. I figured I would need plenty of studs to hold the beam and hold the door hinges. With the south wall and north wall complete, the two side walls were next. Some 45 degree cuts on the top were about the most complex part of the side wall framing. It was a good time to think about the stud placement relative to the outer edges of the north and south walls. Locate the side wall studs so the edge of a full sheet of sheathing can fall on the center of a stud. The second sheet can then be cut to fit. This helps keep down the number of cuts on the sheathing. Assuming the north and south wall are parallel and plumb, it made it easy to build the side walls laying flat on the floor between the north and south walls. The roof framing is made up of 2x4s at 24 inches on center. This framing was a bit more challenging as it required more angle cuts.

Sheathing With the framing complete it was time to get some exterior sheathing/siding installed. I ended up using LP Smartside panels which can be found at Home Depot. This is a pre-primed engineered wood product which basically means it's a lot like osb with an overlay that give it the faux wood look. I chose it over plywood T1-11 because I've seen issues with T1-11 swelling at the bottom of sheds. I also chose it over Hardipanel because I was doing all the construction work alone and just the thought of muscling around 4x8 sheets of Hardipanel by myself gave me a hernia. Each panel has ship-lap edges so installation was very simple. I believe it took a total of 10 sheets.

Collector The hard work of framing the solar kiln collector (otherwise known as a roof) was covered in the framing section. Before the polycarbonate sheets could go on, an additional support framework was put in place. Treated 1x4 appearance boards were used. Why treated? It was the most cost effective off-the-shelf solution available at the local hardware giant. All horizontal boards were ripped down to about 2 1/4" while the verticals were left full width. Two intermediate horizontal boards were included to help support the long lengths of polycarbonate. With the new support framework in place, all of the roof boards including the 2x4 rafters were painted flat black. Anything that would be exposed to sunlight and not get covered by interior sheathing was painted with the cheapest gallon of flat black I could find at the local Habitat for Humanity restore. The support framework was complete and ready for the polycarbonate panels. Two rows of "wiggle foam" was installed at the top and bottom horizontal boards while single rows were installed at the two intermediate boards for additional support. The wiggle foam is sold right along with the corrugated polycarbonate. Using some spray adhesive on the backs of the wiggle foam helped hold them in place until the polycarbonate could be installed. Some standard adhesive backed foam weatherstripping was put down on the two vertical boards on the end. Corrugated polycarbonate can be found at just about any hardware store. It is important to purchase the clear panels that are UV resistant. If not, they will begin to become brittle and may only last one or two years. These panels were purchased at Home Depot. They are made by Palram and are branded as Suntuf. The hold down screws with rubber washers were also available right along with the wiggle foam and panels. On all of the horizontal runs the screws were placed at the corrugated peaks to help minimize any water leaking in. Unfortunately, that wasn't possible on the vertical runs, but hopefully the rubber washers will do their jobs. I predrilled holes at all of the screw locations. The foam weatherstripping on the vertical supports had to have small sections cut out at each screw location before predrilling. If the weatherstripping isn't cut away, the drill bit or even the screw itself will grab the foam and twist it up into a big mess. Obviously this was learned the hard way. With all of the holes predrilled, installation of the polycarbonate was just a matter of laying a panel on the wiggle foam, screwing it down, then laying the next panel on with some overlap. Each panel was 2' x 12' so there was a substantial amount of overhang. I found that the easiest way to trim the polycarbonate was to first draw where I wanted the cut line with a sharpie. Then I scored the line with a utility knife. If it is scored well then you can easily create a starting cut at the edge of each panel with the utility knife then simply roll the excess polycarbonate. It should separate at the score line as you roll it. If it stops separating, don't force it. Pull out the utility knife and go back over the score line to make sure there were no skips. Hang on to the excess polycarbonate. You never know when you may find a good use for it. A ridge cap was needed to finish off the collector panel installation. A polycarbonate ridge cap was available but it was pretty expensive and I didn't see the value of polycarbonate in an area that wouldn't really contribute to the heat gain of the kiln. On the same shelves were some Onduline corrugated panels and ridge caps. The Onduline ridge caps were less expensive so I grabbed a couple of those and topped off the kiln.

Doors The doors were framed exactly as shown in the Virginia Tech kiln plans although the overall width was adjusted to fit my shorter kiln. Sheathing was installed on each door and the vent openings cut out. Finding hinges large enough became a bit of a challenge. The big hardware retailers didn't carry hinges big enough. In the end I was able to locate them at the local Tractor Supply Co. The 10 inch hinges weren't cheap at about $10 a piece, but I hope they'll be able to hold the very heavy doors for a while. If I had planned a little better and had the hinges in advance, I would have included the extra blocking while the doors were being framed. Since it didn't work out that way, I added the extra blocking (doubled up 2x4s) to the doors at each of the hinge locations with ugly toe nailing in some areas. Both doors were set in place and some shims were shoved under the bottoms to lift them up a bit. Each hinge hole was predrilled and lag screws were used to hold it all together. Even with the shimming the doors drag the bottom a bit, but I figure that extra friction just helps keep the doors in place when they're closed.

Insulation Admittedly I dropped the ball on the insulation and vapor barrier pictures. It seems I accidentally deleted those pictures. However, there really wasn't much to see. Standard wall insulation was stapled between all of the wall studs and door framing. Every gap and opening was filled with batt insulation. Plastic vapor barrier was then stapled over the entire interior of the kiln with substantial overlap at any seams. And this is where construction ended for quite a while. Collapsing ceiling plaster in our old house forced my attention away from kiln construction. It was probably eight months or more before I was able to return to the solar kiln.

Interior Sheathing Interior sheathing installation was just a repeat process of the exterior sheathing but with a hotter working environment. For the interior sheathing, ten sheets of 1/2" treated plywood was used. Every attempt was made to keeps gaps to a minimum. Anywhere there was a gap, a healthy bead of silicone caulk was applied. In hindsight I'm not sure if this was the proper sequence since the interior wall coating didn't stick very well to the caulk. It may have been better to coat, then caulk liberally. With the door interior sheathing installation complete, all of the vent opening had to be cut out. The easiest process I found was to use a larger hole saw. Drilling from the outside, the hole saw would rest against the vent opening walls as a guide and I would drill part of the way through. Then from the inside of the kiln and using the center bit hole as a reference, the hole would be drilled all the way through. This kept the chip out to a minimum. With a hole in each corner I could then connect the dots with a small handsaw. Remember, my location doesn't have any electricity so all drilling was with my cordless drill and all sawing was done by hand. The north wall above the doors was sheathed and the vents were cut open the same way the door vents were opened. All interior sheathing was completed and it was time to start coating the interior.

Interior Coating Coating the interior of the solar kiln was a simple but critical step in the overall construction process. The coating serves two very important purposes. The first obvious purpose is to create a barrier on the interior walls that will help keep moisture and condensation from making its way to the batt insulation and reducing its ability to do its job. The second purpose of the coating is to function as collector surface anywhere sunlight may fall on it. This is probably only applicable on the north wall above the doors and the side walls, but the moisture protection aspect is important everywhere in the kiln. Roof and foundation coating (asphalt roofing tar) was the product of choice. A five gallon bucket was just barely enough for a single coat on all interior surfaces. I may apply a second coat if it appears the single coat isn't enough after drying a couple loads of lumber. No fancy tricks to using this stuff. It's thick and messy. I painted all of the surfaces with a liberal coat. After the first coat dried I went back and did some touch up particularly in those areas where it didn't stick very well to the silicone caulk. Don't expect a picture perfect coating job, especially on the vertical surfaces. The completed job in my kiln has very uneven consistencies, but it is black and coats and that's all that really matters.

Vents Let me start this section by stating my vents are clearly over designed and I'm still not convinced they are the best way to get the job done. I attended one of the short courses on solar kilns given by Dr. Brian Bond at Virginia Tech. I recall that he wasn't completely pleased with the off-the-shelf foundation vents they were using on the kiln. Also, most of the other kilns I've seen utilize a top hinged door for the vent covers. I thought the hinged door covers would be a bit difficult to regulate well so I opted to design my own. Essentially they are two horizontal doors that slide on upper and lower tracks. They are easy to adjust accurately, but their downfall is they don't do a good job if we get a driving rain (which doesn't happen too often), and construction is quite a bit more involved. Future plans for the kiln include an awning made from the leftover polycarbonate above the top vents. This should help keep rain out of the vents and even off the back door cracks and crevices. There are lots of ways to skin this vent cover cat. Pick the method that works best for you. The vent covers were constructed from leftover treated 1/2" plywood from the interior sheathing and some treated 1x appearance boards. The first step was to put some horizontal boards inside the vent openings to keep the doors from falling into the vents. I could've just as easily made the doors larger than the vents and let them slide against the exterior sheathing, but I already felt the openings were too large so it wouldn't hurt to fill them some. With all of the doors cut, they were used as guides to locate the upper and lower slides made from the same plywood. Once these slides were in place they allowed for the installation of the appearance boards with a little overhang. This overhang serves to hold the doors in place. Basically this was a "built up" rabbet instead of cutting rabbets into each top and bottom trim piece. Unfortunately, in an effort to get a reasonably tight fit on the doors, many wouldn't slide well or at all. The fix was to rabbet the edges of the doors until they slid easily. So, despite my effort to avoid rabbeting a bunch of boards, I ended up doing it anyway. With all of the custom fitting complete, the doors were slid in. The double doors allow an unlimited amount of accurate adjustment. A couple of the tighter fitting doors get tough to move when it rains and they swell, but hopefully the future awning will reduce this.

Fan Deck Whatever fans you choose to use for your kiln, you will need a place to put them. The fan deck is what supports the fans and helps to separate the heated dry air heading into the lumber stack from the moist somewhat cooler air exiting the lumber stack. Fan decks are one of those areas that aren't covered in much detail in most of the plans you find, so you're left to your own imagination and design skills. The fan deck for my solar kiln started off as an overly complex, way too heavy monstrosity of boxes made of studs and plywood. After completing the first segment of the original fan deck I realized it was just too heavy for the 2x4 rafters to support well. Especially when fans still needed to be installed and the baffle would be hung from the fan deck. Back to the drawing board where I found simple is better. Had I built the kiln floor with 3/4" plywood as I should have, I would have had the offcuts that would work perfectly for the fan decks. I didn't, so I had to go buy a full sheet of treated 3/4" plywood to use. I built the fan deck in three segments; one for each fan. A benefit of building each one separately was that it made it much easier for a single person to install. Each segment is simply a piece of the plywood with a hole cut in it for the fan. Each plywood segment is held in place by screwing it to studs hung from the rafters. There was nothing scientific about where the fan deck was located. It is approximately where the back edge of the lumber stack will be. I have assumed a four foot wide stack with approximately equal spacing front and back. The determination of the exact fan deck location was driven by where the closest stud was in the end walls. With the end wall studs located, I nailed the outermost fan deck supports directly to the walls. Solar Fans This step in the solar kiln construction process will probably be much simpler for most of you than the following process. The empty, city lot where I keep my equipment and do all of the milling does not have electricity. I have very limited space at my house for a kiln and moving the lumber back and forth would be added, unnecessary work. Having electricity run at the log lot would be an added, ongoing expense that I just couldn't justify. The obvious answer for me was to run solar powered fans. Hours of internet searches turned up the same solar attic fans that it seems everyone is selling. Unfortunately, these are priced in the $300 to $400 range. They are typically rated for 800 cfm and run off a 10w panel. Luckily I found an alternative at Home Depot. The GAF Masterflow solar gable fan includes a 10w panel and is rated for 500 cfm (not that I believe any of these manufacturers' ratings) and cost $179. If the flow ratings are correct, then the price per cfm isn't much different between the fans. However, I felt having more fans would more evenly distribute the air flow through the lumber stack. I bought three. The included fan mounting brackets are simple angles that are bolted through predrilled holes in the fan housing. But when they are installed, the brackets extend past the face of the fan housing. To minimize any air gaps, I wanted the fan housing to extend just beyond the face of the fan deck plywood or at least flush with it. The brackets as supplied could mount to the back of the fan deck but that would leave a substantial gap between the fan housing and fan deck. Because of the bracket bolt head and the close fit between the fan housing and the fan deck, it wasn't possible to screw the brackets to the front of the fan deck. To fix this minor issue, I simply drilled new holes for the brackets further back on the fan housing so the brackets could be screwed directly to the back of the fan deck while some of the fan housing fit through the fan deck opening. The supplied brackets for the photovoltaic created another set of minor problems. The brackets were intended to lift the panels up off an already sloping roof. My plan was to install the panels to the vertical surfaces at the front of the solar kiln. There just wasn't enough "lift" to get the panels out to approximately 45° to optimize solar capture. Two separate modifications were made. One to make the brackets functional and one just because I wanted to. The first modification was to remove the mounting brackets, drill a couple new holes in the pv panel frame and remount the panel to the brackets after rotating them 90°. The only reason I turned the panels sideways was to reduce how far they stuck out from the front of the kiln, since the panels are rectangular not square. Yeah, I know, it's a goofy reason to add more work, but I like it better this way. The second modification was to make some new, longer connector bars that span the distance between the panel itself and the mounting bracket on the front wall of the kiln. Using some spare aluminum I had laying around, I carefully and slowly cut out strips on the table saw (man, I hate cutting aluminum on the table saw), drilled some end holes, rounded the corners, and cleaned up the edges on my edge sander. The longer brackets allow mounting the pv panels at an angle equivalent to the kiln roof line. I experimented some by making a couple additional bars of varying lengths. The thought was, I may use the longer bars in the summer to get more adjustment in the panels so they lay almost horizontal and capture more sun. Likewise, the shorter bars will allow the panels to adjust to almost vertical since the sun lies lower in the winter sky. Mounting the pv panels to the kiln wall was a simple matter of nailing some left over treated 3/4" plywood strips to the studs, then screwing the pv panel mounting brackets to the plywood strips. The mounting location on the front kiln wall should allow rain to run from the roof directly onto the pv panels while keeping the wires and attachments under the panels dry. At one point I had considered mounting the panel to the higher portion of the roof, but it occurred to me that there was no good reason for that. Mounting the panels to the front of the kiln made it very easy to install and it makes it easy to access the panels for cleaning. The wiring included with the fans are "plug and play". The pv panels are prewired with an eight foot length of wire and a connector. These connectors mate to a connector prewired to the fan motor. Luckily, there was just enough length in the wire to reach from the front of the kiln to the fan motors..... barely. The panel connector and wire was fished under the polycarbonate and over the wiggle foam to get into the kiln. I then ran each wire on the underside of the closest rafter, up and over the fan deck, and then attached to the fan motor connector. As soon as the single connection was made, the fans started spinning. Collector Plate & Baffle The purpose of the collector plate is pretty straight forward. The solar kiln needs as much surface as possible to convert the sun's energy to heat within the kiln. The black fan deck and end walls will both see direct sunlight and will both serve to create heat. Once the stack of lumber is loaded in the kiln, the top of the stack is a large horizontal area that can serve as a collector to create more heat. The material used for this collector isn't important as long as it is dark in color. Flat black is the optimal color for converting solar energy to heat energy. The VT demonstration kiln uses plywood that has been painted black. For my solar kiln, I chose two sheets of thin, corrugated metal roofing. The logic behind the choice was simple. The roofing was sold in 12 foot lengths so I could cut them to fit the exact length of the kiln, the price was reasonable, the roofing isn't subject to bowing and curling, and they are light which is important when I load and unload them alone. The first step was to cut each of the two pieces to length. The interior length of this kiln is approximately 136", so I cut each piece to within a 1/4" of the overall kiln length. I wanted a good, close fit to minimize hot air bypassing the lumber stack by sneaking around the ends of the collector panels. If you use metal roofing, wear gloves as the edges of the panels are quite sharp. Then it was simply a matter of spraying on some primer and brushing on some of the exterior latex I had left over. I'm sure that as time passes, the panels will require some paint touchups, but I'll keep a can of flat black spray paint handy just for that purpose. The collector plates are placed on top of stickers, which are on the top row of boards. The baffle is anything you use to span the variable distance between the bottom of the fan deck and the top of the lumber stack. The goal is to keep the air flowing in a direction that allows the greatest amount of moisture removal from the lumber. The fans blow air that has been heated under the polycarbonate, towards the front of the kiln where it enter the stack of lumber, collects moisture from the wood, exits the stack where some heads out the exhaust vent and some is drawn up towards the fans to begin the cycle again. The fan deck, baffle, and the collector plate create the primary boundary between the dry heated air and the moist, cooler air after it passes through the lumber. A basic tarp seems to be the baffle of choice for most solar kilns. In my effort to maximize the amount of heat generated, I chose a "heavy duty" tarp from Northern Tools that has a silver side and the all important black side. The tarp is 10'x12' which is almost perfect since the interior length of my kiln is just under 12'. Ten feet was way more than enough height for the baffle, so I folded the baffle in half. A couple extra stickers were cut to length and placed in the fold of the tarp. Some washer head screws were used to attach the baffle to the stickers. Some of the interior sheating offcuts were ripped to a couple inches wide and cut to length to fit the width of the kiln. I then sandwiched the loose edge of the tarp between the fan deck and the sheathing offcuts and screwed them together. I figured this was better than screwing or stapling the tarp to the fan deck, which would eventually create tears in the tarp. The couple inches of excess on each end were left in place and I just fold those back and against the end walls when the baffle is rolled down. The stickers in the bottom of the tarp make it easier to roll the tarp up out of the way or roll the tarp to the height needed for a load of wood. The stickers with some rolled up tarp can be easily tucked under the edge of the collector plate and it holds in place without the need of any additional attachment. With the black collector plate, black baffle, black fan deck and gap fillers, the only thing visible from the south side of the solar kiln is collector surface. Sensors If you are going to have any reasonable chance at controlling your solar kiln, you will need some basic instrumentation. The primary output variable that we're trying to control is the rate of moisture removal from the lumber. Monitoring that variable involves weighing kiln samples and possibly the use of a moisture meter. Those activities are covered in the discussion on operating your kiln. The rate of moisture removal is affected by several other related variables such as air flow rate, temperature in the kiln, and relative humidity in the kiln. With these simplified solar kilns, the air flow rate is mostly fixed based on the fans that are purchased and installed. Temperature can be somewhat controlled by adjusting the vents, but the primary control is whether the sun is shining or not. Relative humidity can be affected by the remaining moisture in the lumber, the outside relative humidity, the temperature in the kiln, etc. but our primary means of controlling humidity is also through venting. The only way to know how the kiln is performing and how you are affecting its operation is to monitor the temperature, the humidity, and ultimately the moisture content of the lumber. The easiest way to monitor temperature and humidity is to grab a low cost thermometer/hygrometer and place it in the kiln. I bought one from RadioShack for around $20. It measures relative humidity from 25% to 95% and temperatures (indoor/outdoor) up to 122°F, although the outdoor sensor is rated to 158°F. I placed the thermometer/hygrometer on the floor at the inlet to the lumber stack and near one of the end walls. The second temperature sensor (outdoor) is on a cable so it is stretched out to measure near the center of the lumber stack on the inlet side as well. With the thermometer/hygrometer on the floor and near the end wall, one of the vent doors can be opened and reading taken without having to open the main kiln door. For the sake of this website (and my twisted desire to track the kiln's operation) I also purchased two dataloggers from Lascar Electronics (see Resources). One is temperature only (costs around $60) and is used to get reference data on the external temperatures. The second is temperature and humidity (costs around $85) and monitors the internal conditions of the solar kiln. These dataloggers take multiple data points (16,000) at predetermined intervals (mine are set for 5 minutes). When it is time to gather the data for review, the dataloggers are simply plugged into the usb port of a laptop and the data downloaded to the computer. Each sensor came with its own mounting bracket and protective usb cap. I modified the protective cap on the temp/humidity logger by cutting the end off. This allowed me to attach a usb extension cable, wrap the connection in plastic and seal with electrical tape. The modified cap and extension cable on the temp/humidity logger allowed me to mount it at the inlet side of the lumber stack and then route the extension cable to the exterior of the kiln, near the temperature-only logger. This makes it easy to download data from both dataloggers without having to crawl around inside the kiln while it is drying.

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