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.
Lessons Learned (which is a nice way of saying I messed up and you don't have to) - First mistake: originally I bought and planned on using 3/4" osb flooring for the upper base surface. It was cheaper than the 3/4" treated plywood and the 1/2" treated plywood. After much thought I decided against it based on the fact the floor may see quite a bit of moisture from condensation. Second mistake: I opted for the 1/2" treated plywood thinking that the thinner material wouldn't matter if the lumber stack would sit across all of the floor joists. Third mistake: later in the kiln construction a temporary cover blew off and rain puddled on the floor which made the thinner floor bow in places. So the lesson learned is pay for the 3/4" treated plywood and keep everything well covered until the kiln is completely under roof.