gotago, you will get a wide variety of opinions on this topic! I have seen pics of chickens playing out in Minnesota snow, and I have also heard stories of frozen chicken legs here in Tennessee.
I think most people will agree that no matter where you are, it is important to keep chickens dry and out of the wind. Most will also agree that you need a decent amount of coop ventilation as well.
Factors to consider with your own chickens are:
* size (banties can suffer from cold more than standards),
* comb type (breeds with large combs/wattles can get frostbite more easily;
* feathering (heavily feathered and feather foot breeds can stay warmer, but you have to watch they don't get ice balls on their feet).
That is a long-winded way to say there is no one temperature that clearly indicates you need extra heat. Some folks in your area probably never use heat, while others will use a lot.
For my spoiled hens, I use ceramic warming bulbs with heat-holding clamp lamps when it gets down around freezing or below. My personal goal is to keep the inside of the coop over 40, and I like 50 even better. The first winter I just watched the hens to see how they acted in different types of weather, and my oldest hen started shivering around 38ish degrees.
I also install windbreaks around the run in winter, and I leave hay bales in there for warmth too.
Hope this novel helps. Good luck!
** Ed. to add link to article from my site on my winter arrangements ~
http://theworldofjenotopia.com/cmsjoomla/index.php?option=com_content&task=view&id=53&Itemid=27