Do you have any air vents in the coop? FYI....From what I've read on BYC it is a must to have air vents, and they should be placed towards the top of the coop for the moisture to escape....even when it's really cold the moisture has to go somewhere. Moisture is created when ducks/chicks breathe (especially in cold weather) and from their poop...it isn't healthy for them if the moisture cannot be ventilated. Also, you probably have already thought of this, the coop needs air vents for the summer that can be supplied by a window, or air vents in one or two walls. You can close off/tape off if necessary the wall air vents in the winter.
Coops looks great by the way!!!!!