I checked my thermometer in my first green house where I have 7 cords. Outside temp of 28 and inside temp of 68 at 11 am. No air flow so it's probably not drying much but shouldn't be any increased moisture content.
It is probably drying. Just the natural low humidity indoors will help it. If your relative humidity outside was around 70% then your natural relative humidity inside at that temperature would be well under 20%. That would certainly draw moisture out of the wood until you reach an equilibrium. Do you get condensation on the plastic or the sides? Do you see condensation in the morning when things cool down? Might be worth it to evacuate the moist air once a day before it cools down to circulate in the cool air that still has low absolute humidity.
There is always a little in the morning. I could probably drop the moisture content of the wood down a little by drawing some air in the winter but I don't know how much of a difference it would make since the sun doesn't shine a ton here in the winter. From April until October I run a box fan in the door pulling air across the stacks from 1pm to 6pm. I have a feeling if I did some tests that getting down to 15% moisture requires much less effort than going from 15% to 10% with this setup. I don't know if it would make that much difference with OWB setup.
