Can't hurt, someone might know. I am running a passive solar firewood kiln. I have eight separate racks at one cord each, the way I have the plastic sheeting arranged it is three separate kilns. 3 cords + 3 cords + 2 cords = 8 cord capacity.
For I think five years now I have been starting with fresh green dripping sap spruce splits in April and May and finishing with 8 cords of just beautiful firewood around 12-16% MC by September. Just gorgeous stuff, like magazine cover pretty. All spruce, my wife is allergic to birch and those are the two trees that grow up here.
This winter, 2019-2020 my first three cords out of the first kiln were run of the mill excellent. I got into the fourth cord in January 2020, the first load out of the second kiln and my wife started reacting to what looked like a trivial amount of surface mold (black) on some of the splits.
I had sold the wood in the third kiln to a guy from my church. It was looking real good in early January, but that kiln was starting to show some black surface mold by late January/ early February too. He closed on a house in December and I was helping him load the two cords he bought one truckload at a time every couple weeks or so, so I was in and out of that one.
I am seeing 60-70 degrees (Fahrenheit) solar gain in the height of summer when I have 18 hours of sunlight. 80dF outdoor ambient I will see mid 140s Fahrenheit inside the kilns with a BBQ thermometer. I have never yet measured +150dF. Typically I can measure above freezing temps and see condensation on the inside walls of my green loads when daytime highs are in the +20s dF.
Ordinarily I load green splits in March and leave the kilns sealed "tight." I plan to open the side curtains to keep the temp inside the kilns under +95dF every year until I get down to FSP, but generally my stacks are moving in the racks (indicating the firewood has crossed under FSP) long before the temp inside the kilns hits +80dF. By "tight" I leave an opening at the roof peak about the size of a cantaloupe for every cord in the kiln. When I roll up a side curtain each cord is covered on top, has a clear plastic wall on the south face and is open to outdoor ambients on the north side, about 32 sq ft open per cord.
I have some pics in my phone, not sure where my data cable is. If you internet search "passive solar firewood kiln" three of the first five images will be my backyard, the trailer for my boat happens to (accidentally) be DOT orange. The 55gal drums as BBQ smokers painted silver, also mine. I am using PT floor framing, a thick layer of plastic to keep groundwater vapor out, then SPF framing above that with the metal screws well sunk since my wood stove has a catalytic converter in it.
I cannot figure out how I had black mold growing in my kilns when ambients were in the -30 to -40 dF range. Even with max solar gain (unlikely with 4-6 hours of sunlight), temps inside the kilns should have been below freezing. I am not sure what to do differently this year, other than maybe move to Arizona.
Have you guys seen black mold growing in subfreezing temps? My plan is to replace all the plastic sheeting and spray the framing and rafters with concrobium, but it feels sort of SWAG at best. I was working on the atmospheric science department at my local university and making nice with my county extension agent, but COVID19 hit before I got anyone on site.
Thanks for any input you got, cordwood for my stove is about half the price of heating oil for my boiler. I use a lot of one or the other.