What were their symptoms? How were they sick?
Chickens usually handle cold weather quite well. The risk in below-freezing temperatures is frostbite, not a disease. As long as you don't have breezes blowing directly on them, the more ventilation you give them the less risk of frostbite. That clears the humidity out of the coop. The high moisture content in the air can lead to frostbite if it is cold enoough.
Another risk of not enough ventilation is that the ammonia in their poop can built to high enough levels to cause them respiratory problems. Birds are fairly sensitive to air quality.
The way I provide enough ventilation during cold weather without having breezes blow directly on them is to have openings higher than their heads and backs when they are roosting. It's really hard to have too much ventilation in cold weather as long as it's above their heads while they are roosting. During the hot part of the year, I open a window at their level and have ventilation near ground level. Heat is more of a danger to them than cold.
