I've heard how hardy ducks are. And I reckon many of them are. My nine runners do better (I'm in New England) if it's below 35F in the walkout basement area that I have made in to a duck pen. I call it the duckie storm shelter. It's about 40 to 45F there in the winter. They go out during the day, depending on the weather (15 degrees and snowstorm, no. Sunny and 25, sure - for a while).
What I was told was that if they have a warm dry shelter with plenty of bedding and not airtight but not too open they'd be fine to at least 20 degrees F. So I built a house for them that is double-walled, insulated with vermiculite and perlite (decent R value and they do not mold or get eaten or burn). They have over a foot of bedding, and a three piece plexiglass drop ceiling. The door is about 18" wide and 2' tall.
They also need plenty of water for drinking and washing their heads at least. Water freezing up is one issue people have. Some have heaters or heated water buckets, some just dump and refill with warm water a few times a day, but it's something to be dealt with.
If I had a barn or outbuilding that stayed 40F I'd probably have them in there, but my place isn't setup like that. The basement is my barn.
A major advantage this last winter with having them downstairs was that I did not have extra shoveling of snow. We had three feet of snow on the ground for several weeks. I didn't have to add shoveling the ducks out to my to-do list. I did open up a few small areas for them to noodle around, and I put straw on top of the snow for them. That worked out well.