Moving pullets outside in cold weather?

I have 13 pullets that are about 6-7 weeks old and appear to be fully feathered out. I live in northern Indiana and our temps have been pretty unpredictable. One day it can be 68 degrees and 45 the next. The extended forecast is showing highs of 40's and lows of 20's at night. Plus its snowing right now! I have them in an unfinished basement at the moment and took the heat lamp away last week. They're perfectly content and running around scratching and being chickens. Not exactly sure the ambient temp down there, but I know it's cooler than the main house but warmer than outside (plus no wind!). At what point would they be ok to go out in those temps? Or should I wait for it to warm up later? I'm hoping Spring decides to show up soon, but I know the girls are tired of the drab brooder walls, lol!

 

I'd feel a little better if the nights were in the 40's, the 20's are what gives me pause. I'd guess that since there's 13, they'd huddle together to stay warm, but still concerned. The roof of the coop does has a radiant heat barrier tho. I'm probably being an overprotective new chick mom, but don't want my girls to freeze!