I understand the importance of ventilation but on nights colder than -10F I cover the vent. They always have clean dry pine chips and with our 5 ducks I haven't had any moisture problems doing this, of course the door isn't exactly airtight. We have -20F this morning so it will be 9 or 10 before I let them out, they will have a pool for a few hours which they love, pine chips on the snow to protect their feet. This is how I have done it for years with no issues. Oh, inside the duck house it is 18 above at the moment.