Do they roost? They really shouldn't need heat unless they are very young. The problem with heating a coop is you will have a real problem if you ever loose electricity and thus heat after getting them used to being heated. That said. The easiest way to give some warmth without a heat lamp is to put a brick in the oven, heat until its hot all the way through and then wrap it in a towel or burlap, or something and set it on the floor of the coop. The brick will hold the heat for a while and as it cools the chickens will just move closer to it if they want the warmth.