I am in the process of building a new home in Maine. The house is to be done in two phases. This year the foundation was poured and the floor system finished(beams, joists and sheathing). I covered evrything with blue tarps. To prevent the foundation or cellar floor(full cellar under the entire footprint) from cracking from the cold, temporary electric heaters are being installed. When I went to check on the house(the first time in two months) this weekend, I found mold and drops of water on the underside of the sheathing. The cellar doors and windows were closed tight and it has been one of the wettest Octobers on record. The top of the sheathing was dry. I sprayed and sponged the mold with bleach and this got rid of almost all of it. I’ve left the windows partially open but this won’t be possible when it gets cold.  Is this mold the result of a tightly sealed basement or do I have another problem. I had planned to insulate the floor but wasn’t sure what to do about a vapor barrier. Any suggestions? I think the insulation will be becessary to keep the cellar warm enough.