Update on the subject. After reading all of the advice people so kindly shared... I opted not to provide any heat in my coop. I have to state that I do not think humidity / moisture is an issue, as I have adequate ventilation (described in an earlier post) and there is not one stitch of frost or ice on the insides of the coop windows. When I did have moisture earlier in the year, I saw it on the glass.
My ladies got frostbite. Only my poor leghorn Pearl has it badly. Her comb is just SO BIG. I have been keeping chickens in Vermont for over 5 years, using two different coops and have concluded that -20 degrees is just pushing the limit. Not only are her comb and wattles frostbitten, but the back of her comb is actually frozen. I discovered this when I was massaging Vaseline onto it. When ordering chicks, I assumed that she would have a rose comb, but it wasn't specified. My mistake.
Factors: I only have 4 hens in a 4x6 coop. Pearl has a single comb. There is cold, and then there is twenty below zero.
The next day I went out and bought what I needed to set up a ceramic bulb (porcelain socket fixture) and put it in the coop. It doesn't "heat" the coop, but kept it closer to 10 degrees above. I know that there is a risk of fire. I totally support the fact that down into zero degree weather, chickens can take care of themselves given appropriate housing, but at -20, there may be a need for a little help - especially if you've got a single comb bird.
I will only turn this heat source on when it is significantly below zero for a substantial amount of time.
I used to use a red brooder bulb, but this flock gets aggressive and pecks Pearl if I light them all night.
Now on to my project of rehabbing my leghorn!