I purchased a seed warming mat to put down on the floor of the coop. I just throw the pine shavings on top of it. It is about 48 inches by 20 inches and generates about 100 watts of heat. http://www.amazon.ca/Jumpstart-48-by-20-Inch-Seedling-Heat-Mat/dp/B000E3B8XG I thought it would help keep the coop warm. It likely makes no difference to the air temperature of the coop due to the amount of air flowing into and out of the coop through the vents. I was thinking of removing it as it seemed like a waste of electicity. However, I noticed all the chickens like to stand on it. It likely warms their feet.