As long as their coop blocks some wind, they will be okay in the cold. Don't forget, unlike chickens your ducks are wearing little down coats all the time! I live in East Texas, and it has gotten down to 15 degrees at night this winter but my ducks are fine. No heat lamps. The coop has solid wood for the bottom four feet then 1/2" wire to the roof. Make sure there is lots of bedding for them to snuggle into and that the wind is blocked. Don't put water inside the coop with them. They'll be fine without it overnight; just go out in the morning and break any ice off their water so they can drink. And, of course, since they don't have any water don't leave any food in the coop overnight. When it's real cold here I got out in the afternoon and throw out some chicken scratch so that digesting that will help keep them warm overnight. Ducks are VERY hardy! Remember, in the wild they swim around in freezing water all the time and nest on the banks of lakes and stuff.