My ducks are fresh air fiends, and I have read that their little lungs are susceptible to problems from too much damp, still air (which probably contains more in the way of mold spores and possibly ammonia fumes).
The entire top of their spring/summer/fall house is open, covered with half inch hardware cloth to keep predators out. I did add a plexiglass three-panel drop ceiling that can be opened or closed depending on nighttime temperature, but their sliding door is always open and the drop ceiling isn't air tight.
ETA And their house has an attached screened in porch.