As long as they are healthy, ducks have a reputation for being very hardy. They do need some protection from nasty winters, though.
If you haven't read Storey's Guide to Raising Ducks, I recommend it.
You need to keep an eye on them, especially when it gets bitter cold. They still need water for drinking and washing their heads, even in winter. They need more calories. They need to be out of the wind, and in an insulated house.
How many ducks do you have? If you have just a few, they cannot stay as warm as a larger flock can by huddling together.
By the way, my runners have never huddled together a day in their little lives, so winter will be interesting here in New England.
This is their first year, so we will see how it all works out. I am using the deep litter method, where I am letting bedding build up (I clean the worst parts off, but there are some droppings in the bedding) so that it will insulate the floor and provide just a little heat from the breakdown of the droppings. There are some discussions on BYC of this method, sometimes just called DLM.
I am also looking into adding more protection to their house. Since it is summer, my main worry is shade and air flow. I think we have that covered. Come autumn, I will probably add some panels over some of the vent windows, and perhaps something across the ceiling area, which is open but covered with hardware cloth. I want something that will let light in, so they don't have to live in a cave.
Good that you are thinking ahead!