We get plenty of snow! Luckily this past winter was fairly mild, but typically it's lots of snow and single digit nights. I know lamps are typical for heating coops-but when I was a kid, a barn I boarded at had a massive fire from a heat lamp for chicks, so I'm so nervous about it. Ack.
I recently saw one member in the snow with a coop completely open with mesh across the front. I don't think you need to keep adult chickens warm. The feathers are insulating. Babies hide in under mum, don't they?
Anyway, no need for me trying to scrape my memory banks, it's not something I needed to figure out. Heat is my bugbear. You'll hear from the "locals" who can give you specific advice about it.