If they got their feathers and you can keep the breeze / draft / winds off them, they should be ok. It would not hurt to put a heat source in there too if you can. What I have done is taken an old sheet, or in my case, coffee bean burlap sacks and covered the openings to help keep the heat in, then come morning time, remove the covering from the windows etc once the temp comes back up a bit again.
The heat source could also be a small heater on the outside, blowing warm air into the inside. just remember if you put ANY sort of hot something in there, make sure they won't knock it over or start a fire with it. At one time I used to have a light bulb way up in a corner but one of the dunces managed to break the bulb one night jumping up onto the roost, so ... so much for that idea. Just remember if there's a way to break it, they'll find a way.
The temp is not too terrible of an issue but the wind is your greatest concern right now. IF it's a calm still night they can keep warm,but if it's windy and howling through every tiny crack in there, not so much.
Aaron