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 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.
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.
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.
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.
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.
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.
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.
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.
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.