I am building a cabin myself on the weekend, so construction is progressing slowly. The cabin is located in the Pocono mountains and it has a full poured-in-place concrete basement. Over the past 4 months we have built the entire first floor and we have installed Warmboard radiant heat subfloor panels (1.25″ plywood with groove pre-cut in them) over the entire floor. We have learned that though the Warmboard panels can overwinter, the pre-cut grooves collect most of the rainwater that falls on them and channels that water into the gaps between the subfloor, where it drains into the basement and onto the floor joists. We have tried covering the floor with tarps, which has helped but hasn’t fully solved the problem. It seems that the underside of the subfloor collects a lot of dew in the mornings and so they’re wet most of the time. We are coming up on winter and plan to stop construction over the winter months; but we noticed that some of our floor joists seem to be getting moldy with black stains. My question is: is this relatively normal for a house that is under construction, or is it something about which we should be worried?