Lead acid cells will freeze at different temps depending upon the state of charge so it is hard to answer specifically so here is Trojans chart. A fully charged battery would be tough to freeze but at half charge in many areas of the country a battery could be in trouble.
http://www.trojanbattery.com/pdf/WP_...orage_0512.pdf
http://www.lifelinebatteries.com/manual.pdf Page 22 shows freezing points for Lifeline AGM. It's better but is not immune.