They are much hardier than you might imagine.
It's best to always give them a choice (in or out). If you absolutely insist to keep them in for the day, only do so if you have at least 3 square feet per bird.
It's the moisture that will hurt your bird, not necessarily the cold. Keep her dry and you should be golden. Here's some tips.
Here's some other winter management tips...
Water
- Make sure they always have it and it's always unfrozen.
- Empty the waterer at night or put it in the basement (they don't need it once they perch).
- Change it out during the really cold days to prevent freeze or get an electric water warmer
Food
- Food helps them stay warm. Try feeding them a little extra before they perch at night to fill their crops.
Housing
- Make sure there's lot of ventilation above their roosts
- If possible have openings, windows, doors etc... open to the sunny side and everything closed elsewhere to protect from wind.
- Deep litter the floor. At least 4" of "brown" carbon material like straw, hay, wood chips, shavings etc... Ad more material as needed to cover manure and keep the smell down. Harvest the bottom layers in the spring for compost.
- Make sure it stays dry. Keep the waterer outside if possible, cover manure daily (manure is 85% moisture)
- Cover any windy side slits in the coop
For more on raising chickens with nature check out
http://www.abundantpermaculture.com.