I have a new mold issue in my firewood kiln

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.

do you use a fan in the kiln? sounds like you need some air movement.

There is a lot of discussion of solar kilns on forestry forum. You might want to go there.

I do know that mold reproduce by spores and once the kiln is contaminated it might have to be cleaned , it might be just to spray bleach to kill the mold and then refill wood, mold spores are in the air so it might be something that needs to be done in between loads. I believe that they can lay dormant for a long time until moisture gets right for them. this is a guess!

Sounds like you need air movement the mold started when you sealed it up and do more research

Well, I got mold again. Last spring, spring 2020 I pulled all the plastic membrane off my firewood kilns and took it to the dump. I sprayed all the wood framing down with Concrobium. 3 or 4 gallons of the stuff, left it in the sun for three days, got rained on, let it dry, sprayed it with another 3-4 gallons of concrobium, left the kiln framing in the sun for three days and then got all new plastic membrane on before it got rained on again.

Then I loaded up another 8 cords of spruce splits. I left the side curtains open on both sides, so my cord wood wasn't getting kilned. Ambient in there was the same as the surrounding outdoors with good airflow, it was basically top covered to keep the rain off until I got it well below FSP (~30% MC wet basis for the white and black spruce that grow up here). Once the stacks were all moving pretty good, demonstrating below FSP by shrinkage, then I rolled the side curtains down.

Pic of bare kiln framing is May 6, 2020. The full unit to the right in the pic was too moldy for my wife to have in the house, I sold it off, cleaned as above blah blah. Looking at my wood delivery log I started loading on May 16, had my 8 cords stacked on June 2, 2020 and was well below FSP on 06-21-2020. When the first frost hit in the 20s of August I was at 15% MC. The woodstove is running fine. When I split a big piece open and stick my pin meter in the freshly split face parallel to the grain I am seeing 14-15-15-15-15-15-15-16. I am convinced the mold spores are coming in on the cordwood. Already buying next winter's wood from a different vendor, but they are all cutting in the same forests.

Maybe I will try bleach this spring before I load up.

Data mining for the hypothesis. In years past I loaded the kilns over the winter as they were emtpied, had all my new green wood for next winter up in mid March. I don't recall what happened summer 2019 or the preceding winter, I think one of the kids was sick - but anyway summer 2019 was the first time I loaded the kilns with a wheelbarrow instead of dragging a sled over the snow out in the cold and the dark. It went really fast, I was stacking in a cord in 2-2.5 hours instead of two weeks of free time using the sled.

But the two years I loaded in warm weather with a wheelbarrow are also the two years I have ended up with mold. I am going to load at least two of my three this winter without changing the plastic membrane and without spraying bleach and see what happens.

Have you thought of getting an outdoor boiler? I know it is a big expense, but it will pay for itself over time.

Not related to firewood, but I unfortunately have personal experience with mold. Other posters are correct -- mold is everywhere, and usually not an issue, but if the conditions are right, it will grow out of control. You can't see it until you already have a problem.

Mold needs moisture, heat, and a food source, that's all. And if the materal you're trying to clean it off of is non-pourous, you can use bleach. But if the material that exhibits mold is porous, you'll need special chemicals to treat (not sure if they are available to non-mold-remediation folks). And unless the conditions change (humidity/temp), it's not clear to me how you'll get rid of it.

Scott, mold grown inside a kiln primarily occurs when the RH% is high in the kiln, and the temperature is between 85 - 110F. Drying does not occur very quickly in temps below 70F. Mold primarily develops when the wood is still at a high MC%.

To avoid mold growth, you need the temps to be below 85 or above 110F, and the RH inside the kiln to be lower.

When you load the kilns during the summer you are right in the middle of the peak mold temp season. If you simply shift back to loading your kilns during the winter your problems will most likely go away.

Yup, the hypothesis gets stronger.

I just today got into my last two cords of dry wood for this heating season. I loaded them in Feb 2020, and no mold.

I _think_, as Scott Smith has pointed out, when I load the firewood kilns in the winter I have the load down under FSP well before the temp inside the kilns gets warm enough for mold to grow. Today is a perfect example, outdoor ambient ranges from -1dF to +10dF, but I am already getting enough solar gain to start pulling free water out of the sap tubules of the cordwood for next year I already have loaded. The last of the free water will likely be gone before all my snow is melted.