After much hand-wringing and reading recommendations both for and against my plan, I moved my 5 week old chicks out of the basement and into their unheated coop. They spent their first night outside in an unheated coop with the temperature dropping down to 48 degrees F, and you know what? They did just fine, thank you very much. I did check on them around midnight and they were loosely huddled together, but not chirping as if in distress. This morning as soon as it was light they all rushed the feeder and waterer. I guess they didn't move around much last night in the dark. I also made them some oatmeal to warm them up which they seemed to enjoy given the amount of oatmeal stuck to their faces, each other, and the walls of the coop. So this just goes to show that even young chickens can handle the cold just fine if they have a non-drafty coop and a few companions to huddle with.