With metal, you'll want to make sure you do have enough ventilation to prevent condensation collecting, but make sure there are no drafts able to blow on your birds at night while roosting. Personally, I'd want to line the ceiling/walls with something, as RH suggested. Many inches of bedding is good. I'm not familiar with the hardiness of polish, but most breeds can survive with few issues down to 0 degrees, and many have chickens in colder winter temps. than that. Depending on your numbers vs coop size, you might do a search on huddle boxes - I'm guessing your winters are going to get a lot colder than the 20s...lol.
Some folks do add a lamp or other small heating device to their coops when temps plummet, but chickens can handle very cold temps. without that extra if your coop is draft free and liquid water and nutritious food are readily available.