My chickens are in 100+ year old wood sided house that's 8x10 and not insulated in any way. It's definitely seen better days, but I have patched it up best I can to keep them safe and comfortable. There are 2 lift-up windows that I intentionally leave lifted a tad for good ventilation. I use 2 heat lamps that are well-secured with metal straps/bolts to hang below the rafters. I have them wrapped with chicken wire so the bulbs can't be broken. Our temps dipped well below freezing this past week. The coop was not warm by human standards, but far better than nothing. This was the first winter I found them huddled together on the worst days.
The most important things to remember is to provide enough ventilation to prevent moisture in the coop. Moisture is your enemy. It can cause health issues as well as frostbite.
If you do use a heat source, be sure that you don't use extension cords that are not rated for that kind of appliance, don't place near any combustibles such as straw or wood chips. Heat lamp hardware is notorious insufficient and can easily fall -- especially if you have a chicken trying to roost on it, so placement is important.
I remove that hardware and secure a metal strap around the bulb base using bolts and then bolt the strap around the rafters. This has worked well for me for the past 5 or 6 years.
Edited to add ** I do not use a heated waterer any longer. I had one in the pen and one in the house -- both of them melted on the underside where the cord comes out from the element. They were those large green heated bowls commonly sold by TractorSupply and other feed stores.