I haven't a clue really but I wonder if you wouldn't be better off filling the space with concrete or gravel or something like that to store heat from the sun...Even milk jugs filled with water or drums of water...

Here's some info.....

Here is a link that might be useful: Build A Freeze-Resistant Stock Tank

How big of tank? What kind of animals are you watering?

I water horses in a shed, with 5-gallon buckets hanging from a wall, twice a day. They freeze, of course, and I dump out the ice.

I water sheep on the ground, also with 5 gallon buckets. They freeze, too.

If there is no snow on the ground, the sheep buckets freeze first, and have thicker ice. Obviously, the horse shed gives protection from cold.

If there is a foot or more of snow on the ground, the horse buckets freeze first, and have thicker ice. The snow insulates the sheep buckets better than the shed does.

So, I believe that an insulated container could provide a lot of protection. Based upon how well the snow works, I believe you will not have much ice developing, and the ice you do get can be scraped off the top, or dumped out easy.

Your container needs to be easily separated from your insulated box, so you can dump ice, or clean, or refill easier.

Horses are hard on anything, and might likely destroy an insulated box. That's one reason I asked what animals.

Thanks for all the info, especially the link to how to build a freeze-resistant tank. That's exactly the sort of thing I'm thinking of. But I guess if I did a 35-gallon barrel, I'd need to build 2 of them. Jeffrey, it will be for 2 horses to start with. I figured foam isn't that heavy, so it should still be easy enough to dump the water to clean them out.