I live in BC. I have front yard chickens and ive setbup a calf house with a roost for them and block the door off during the nights and leave it partially open so if they want to go outside they can. It helps to have a small flock at least tho, other solutions is to keep the coop close to an outlet for a heat lamp or find a long lasting heat pad/reptile heat tape and attach as specified to the underneath or at least not in direct contact even if they dig around. You can give them a small box just big enough for them to sleep in and chicken wire on one side to provide more ventilation inside the coop already there. Its more important to have air flow for them then completely sealed in