The Hypotheses that work just fine for me so far.
I am going for gracious here. I recognize this thread is text dense. If you disagree with my methods I invite you to build your own kilns, run them your way and let us know how it works out for you.
1.
Capillary action In a live tree there is sap in the tubules. In above freezing temps with thoroughly thawed wood that isn't frozen on the inside capillary action and good air flow (with dry bulb temps less than +95dF which I'll get to shortly) takes about two weeks.
Two weeks, all the water in the sap is out of the tubules, the splits are at FSP or fiber saturation point and the stacks start to move as the individual pieces change size. It's very simple. If the stacks are moving you are below FSP. If the stacks haven't started moving yet there is still wet sap in the tubules. NO discussion, it's a definition.
If you run the temperature inside the kiln up above ~+95dF while the splits are above FSP you run the risk of shrinking the ends of the tubules shut and trapping wet sap inside the split. Every year, without fail, we get a couple or three new threads in the wood stove section "I bought kiln dried wood, it lights good but doesn't burn all the way through. I finally split one open and it meters 35% MC on the inside. Did i get ripped off? What's wrong?" And within a day or two someone with similar experience will say they 'kiln drying' that happened to the wood was just to kill parasites so the wood could cross jurisdictional lines for retail sale, esp common in the NE US.
If you want to make little burritos out of all your splits, a wet wood filling with a dry wrapped wooden tortilla, you go right ahead. I ain't got no time for that mess.
You may look up the FSP of whatever species you are wondering about here:
https://www.google.com/search?q=fib...0j69i57j0l4.4036j0j7&sourceid=chrome&ie=UTF-8
2.
Relative Humidity Calculation To figure out how dry the wood in the kiln is, or is going to be, the operator needs to know both the temperature inside the kiln, and the relative humidity inside the kiln. Having BOTH pieces of data will allow anyone to look up the Equilibrium Moisture Content the splits are tending towards, EMC is point three.
One way to accurately measure RH is with a wet bulb thermometer. I don't have room inside mine to do that. Plus it is hot and humid in there and spinning the thermometer in there for minutes at a time every day is not going to be fun.
Another option is to buy a hygrometer. The cheap ones are calibrated for typical household ranges. They don't work well, and don't work very long in rain forest like environs.
The third option, and one that works "good enough" on my lot is to recognize, with a broad brush, the vapor carrying capacity of air doubles with each 20 dF rise in air temperature. Thus a volume of air at say +50dF and 100%RH, inside a tank, or a baloon or what have you, will, when heated to +70dF and no vapor exchange with the rest of the atmosphere will have an RH of 50%. Same volume heated to +90dF will now have a RH of 25%.
It is the same effect as letting cold "dry" winter air into your heated home. Say your house is +70dF and you are maintaining maybe 30% RH with a humidifier on your forced air furnace system. Outside is +10dF and the RH of the outside air is 70%. Your kid opens the door to watch the mail man work the door on the mail box. You get a big slug of 10/70 air in, and the open door lets a big slug of 70/30 out.
When that cold air gets heated from +10 dF to +70dF it's vapor carrying capacity doubles on the +10 to +30 jump, and on the +30 to +50 jump, and doubles _again_ on the +50 to +70 jump. Doubles three times. it was 70% RH when it rolled in cold, gets cut in half to 35%, gets cut in half again to 17.5% and gets cut in half a third time to 9.25% RH when it finishes warming up. So it feels pretty dry.
Same thing in the kiln, except we are starting with "cold dry" air in the back yard at maybe +80dF and 80% humidity, but using the kiln to jack up the temperature and drop the RH.
https://en.wikipedia.org/wiki/Relative_humidity
https://en.wikipedia.org/wiki/Psychrometrics
Equilibrium Moisture Content Once we know the temperature and RH of the air inside the kiln we can easily look up the EMC all the splits in the kiln are tending towards via diffusion. The bigger the gap between EMC and actual MC, the faster the wood dries. As it gets closer and closer to EMC the drying goes more and more slowly.
I posted a lovely table less than a year ago, it is on page two in this thread, post #30.
https://www.hearth.com/talk/threads/solar-cord-wood-kiln-operation.152699/page-2#post-2105773
I f we knew the temperature and RH in splitter's backyard we could easily calculate the RH inside his kiln, knowing the temperature inside the kiln is +110dF. Today in Woolwich, NJ it is +82dF and 38%RH. So looking at the table that you just opened in a new tab you can see that if it was just top covered the wood in his stacks is tending towards 7.6% MC, because that is the EMC of wood for +80dF and 40%RH.
But with solar gain the inside of his kiln is +110dF. And he isn't pumping water vapor in, other than what is coming out of the wood, so if we look at the EMC for 110dF at 20%RH were going to get a high number. If we look at the EMC for 110dF at 10% RH we're going to get a low number, but the range is 2.2 to 3% MC for the wood to be at equilibrium with the air in the kiln.
That wider diffusion gradient he has created by lowering the RH and raising the temperature around his stack is going to make the stack dry faster than it would if it was just covered on top.
This seat of the pants rule of thumb SWAG is good enough. Does the kiln need "some" air turnover, yes. Does it need a lot of air turnover? No. A cord and a half, two cords like he has pictured above are going to do just fine with a couple holes at the roof peak, one at each end, each about big enough for a cantaloupe to pass, maybe 6-10" diameter.