It is important that they have adequate ventilation without having drafts (wind blowing on them). You might try to fill in some of the cracks in the lower part of the coop and make sure there are not that blow directly on the when they are roosting. I am thinking they will be fine so long as they have each other to cuddle against if it gets too cold.i just got my 4 hens this year (rescues, never had chickens before and i only have 4) and i converted a wood dog house into a coop and they have an attached covered run. THEY FREE RANGE so they're out at least 5 hrs a day.
i'm in Western Washington and the winters are somewhat mild compared to the rest of the country but let's say the average night is in the low 40's/ and sometimes high 30's and occasionally we get 2 week cold snaps (high 20's).
I am very concerned about ventilation and drafts. Also even though my run has a roof and a gutter we put up, it still gets WET inside the run and the floor is concrete so i can't see adding straw or pine shavings just to get soaked. i did put a bit of plastic up on one side of the run that helped a bit but the rain (IT RAINS A LOT here, LOL) just comes in through drips off the fiberglass roof and BLOWS in so... yeah i have no idea what to do about that--they're only in the run for a few hrs a day anyway but i hate it that it's somewhat wet and their feet's on concrete, i do have a childs swimming pool full of dirt in there that they camp in but that's getting wet, also ugh.
But here's the coop: it has 2 large doors (used to be a dog house) and one of the doors i have a board covering it but it still gets air around it as it's not nailed down and the other door is open. Their roost is on the wall that the covered door is on
I was thinking of putting plastic on the open door but i'd have to leave the bottom open about a foot for them to go in/out of the coop
Obviously the coop/run is cleaned out and the coop is full of pine shavings and has roosting bar and nest box, this was right before i did all of that but the door on the right has a board now in front of it (air still does get through the sides) and the left door is open