I have a 20x8wx9t coop. It's a prefab originally designed for mowers ect but was barely used so we turned it into a chicken coop obviously. It's a great set up and is working well for us so far however I think it might not have enough ventilation. A few weeks ago it rained for 3 straight days and at the end of day 3 the temp dropped drastically from where it had been. 2-3 days later the coughing and sneezing started. I confined the few affected birds in the warm dry laundry room for a few days on antibiotics. They made a significant turn around but not before it started spreading through the flock. If your saying MG in your head your right. After reading an article in Backyard Poultry I'm starting to think my husbandry is to blame. The article talked about ventilation in a little more depth than I had given thought to. I do have a heat lamp because I worry about the 8-10wk olds and figured it would be the spot they would go to and try to dry off a little. Apparently the heat lamp will cause them to not be able to bear the cold as well. I fear that although the litter is changed no less than every 2 wks, there are no holes or leaks in the roof that the moisture from the wet birds and having only a 6x6in vent at each end of the coop coupled with the heat lamp kicked in the MG thats present in all back yard flocks including mine according to the NPIP. So with all that said, is the 2 6x6 vents enough? I have a crank out window on the coop but heard that birds take the full brunt of the wind better than a slight draft. Not sure how I should rectify this properly. Your guys input is greatly appreciated.