NO HEAT LAMP FOR CHICKENS. Chickens are not humans and do not need to be "warm" like humans do. This is one of the most common reasons folks burn down their whole coop chickens and all. Chickens have survived for THOUSANDS OF YEARS without being kept in coops by humans. How do you think they got through?
Keeping your chicken flock dry and out of drafts is important. As long as they have shelter from wind and rain or snow, they can thrive.
Their feathers are suited to living outdoors, not in a heated enclosure suitable for humans in cold months.
People assume since the people are cold, their chickens need heat and need to be kept warm. That is not a chicken's natural environment and it weakens them genetically if you make them dependent upon human-provided artificial heat sources where in the wild this would not be available. .