I went to Wilcox Sawmill in Rusk Texas. I have a commission to make a cypress headstone for a former Forest Ranger. I had never been to this sawmill before and man did he have some lumber. The slab the customer picked is about 24" wide at the base and is about 8’ long. Only about 36" will be used.
If I place wood in my container, I get water running down the walls and twisted moldy wood. All of it was cured prior to going in. I put a dehumidifier in with the drain running outside, that helped for a while.
Depends on where you live. I would suspect a leak. Shipping containers usually are a 1 way trip and they are just welded steel. So there is plenty of places for it to leak. You could put some metal paneling on top with an air space to keep the container from getting so hot in the summer. They also make asphalt roof sealer you can swab on top.
Good old WA state, and I also moved to the shadow of Mt Rainier.
The container I bought had only one trip across the ocean, pristine condition. Yes I placed it out of the way, so it is in a shady spot. I picked up some 6 inch fans on market place. Going to put some into the vents to suck air out. Think I can get enough solar to keep them spinning this winter.
As winter approaches, I have the dehumidifier helping. Restacked the wood to hopefully take care of the warp.
You need a way to remove the moisture that comes out of the wood. Fans or the dehumidifier you mentioned. Also if its in the shade it can have temperature changes that will affect moisture in the container.
