I live in Northern Colorado and we just had a crazy storm that dropped 10" of snow on us and seriously cold temps! i was so worried about my chickies and searched these forums for hours looking for an actual solid humidity number to aim for. I did not find anything concrete.
What I can tell you is that my coop got down to 11* and 54% humidity and my girls were perfectly fine, not even a touch of frostbite. Actually, they didn't even appear cold, no fluffed up feathers, no sluggish movement. They were a LOT more hardy than I expected. Our winters get a lot colder than that (think week long temps below 0) so I plan on getting a heat lamp but we have another snow storm and teen temps forecasted for this Wednesday again. It is nice to know I don't need to worry about my girls.