I hate the thought of a heat lamp in a dry coop. There's just way to much risk of fire.
Also if you get a bad winter storm and the power gets
knocked out for a couple days your chickens may freeze.
A cheap way to
help your chickens out in the winter is to use a large heavy plastic
tote with a lid. Cut a hole large enough for your chickens and they will
use it on the coldest nights and stay nice and warm.
This method has been used in Canada for years. No heat lamps were needed, and there was not any
frost bite either.