That roof is two layers of polycarbonate. The second layer provides a little insulation; the walls and floor are heavily insulated to keep heat in. Opening the door,
you can see that about six inches below the polycarbonate is the heat absorption panel of plywood painted black. The sun heats this airspace which triggers three attic fans at the peak. These blow the warmed air down and through the stack of wood(red arrows). This warmer air draws moisture from the wood and recirculates back up (blue arrows) to be reheated. Vents on the back of the kiln allow moisture exchange with the outdoor air. We can dry a full load from 12%MC to 7%MC in just a week or two.
We help steer the air through the stack with wind proof fabric carefully draped. This stack of white oak has a few more rows to go until it reaches the height of that reddish horizontal 2x4 on the left.
Well, that's it. Plans from Woodweb . Our only innovation was the fabric, which works great.