I am by no means an expert, but at 9 weeks the should be getting used to cooler temperatures...I'd think 30's to 40's. Zero is cold, though, and I'd certainly be fretting overnight with my girls. I keep my thermometer above the heat lamp, so I know the ambient temps in the coop overall, knowing that if the girls are cold the temps beneath the heat lamp are much warmer than the rest of the coop. I have also read here and in several sources that draughts can be a concern, so I'm careful about what windows I leave open and when. Unless I know its not going to get below 45, I won't leave the window near their roost open because I know they sleep there. In the last few nights it has been 48 degrees at the lowest in the coop...I have a remote thermometer that I keep next to the bed so I can check frequently. Again, I'm just a beginner...so please add that clause to all of my comments.