The wind chill shouldn't really be a factor in looking at temperature. So long as they have a sturdy shelter with no drafts and plenty of space inside to keep them from being overcrowded should they have to stay in there. The wind chill is only an issue if they are in the wind. And if a chicken is not smart enough to move out of the wind and into their dry house then....well not a chicken with keen enough survival instincts for me to want to breed. The wind chill shouldn't even come into play when talking about coop temps. Plus, it's never that cold here. -53 with the windchill would be record breaking cold. Our average temps are -21 to -26 I'm December to end of February. I avoid wind chill issues by stacking hay bales up all around their run so when they are out and there is a windchill they are protected from it. We get strong northerly winds and our whole property is wide open. Windchill is not an issue.