I posted this a coupke days ago in another thread of mine about molting and cold weather concerns:
Yay! After a fair amount of searching, I found some definitive input about humidity, hope it is ok to post like this here cause I want to save it (and share it):
Yes, you CAN get indoor humidity lower than outdoors, if your indoor is warmer (I am not advocating electric heating necessarily -- there are LOTS of things that can act to make your coop warmer than the outdoors, esp. at night. See 'cold coop' page in .sig below for that subject). What matters in terms of frostbite is relative humidity. For a given amount of water vapor in the air, relative humidity is lower at warmer temperatures.
Also, in terms of temporary weather 'blips', it sure seems to me like having your coop start out pretty dry -- dry wood, dry shavings, nonpooey, nonhumid -- seems to create a sort of 'humidity sponge' that can even out temporary swings in the weather.
Commercial chicken barns seem to aim for around 50-70% humidity. IMO for backyard flock purposes it's when you get to 75-80% that you start getting a bit iffy, and I'd say above 85-90% humidity is really courting trouble.
HYGROMETERS ARE NOT USUALLY ACCURATE right out of the package; you need to use something like the salt method (see 'incubating and brooding eggs' section of BYC forum for instructions) to figure out how wrong yours is so you know how much to add or subtract to its reading. I would not suggest believing a hygrometer reading otherwise
Cracking the doors is probably not enough ventilation, and is not a good location for it in wintertime. See my ventilation page (link in .sig below -- sorry to keep doing that, but the whole reason I did those pages was so I don't have to type all this stuff over and over and over ) for details as to construction, location, size, and management of vents.
Good luck, have fun,
From @aart :
Humidity can't be lower in coop than it is outside, I try to measure both.
Hygrometer is easily tested:
Hygrometers can be tested by putting 1/2 cup salt and 1/4 cup water in a dish or jar and mixing it to a slurry.
Put the salt slurry jar and the hygrometer next to each other in a large sealed plastic bag.
After 8-12 hours the hygrometer should read 75%... note any differences and you're set.
From me: I am "calibrating" hygrometer now.
ETA: The hygrometer was 5 Percent below 75% for outside and 4 percent below 75% for inside. I marked the weather station with a sharpie.