Do they have the mother hen to keep them warm? Mine are also 8 weeks old and have been with the rest of the flock for several weeks now. They are in and out of the coop with the rest of them. All but two of them get under the mother's wings at night. Those two roost together on a ledge over the "people" door, as high as they can get. The other members of the flock are roosting in groups but seem to be handling the lower temps (not below 40 degrees yet) well. My coop was formerly one of those wooden garden sheds so it isn't insulated. It is well-built and tight in spite of it's age.
If you use a heat-lamp, just be careful. They can cause a fire.