I think if it's that cold they will be fine at 64%. Brrrrbb.b. Dang!I am having very good luck with my 3 gallon metal water fount and metal base heater. I have my water in my coop. The coop has about 6 inches of wood chips deep litter. At first, the chickens were scratching stuff into the water. But I bought a patio paver block, put the metal base heater on the paver, and then put the water fount on the heater. The additional height was all I needed to stop the chickens from scratching litter into the water.
Did you actually take a measurement of the humidity in your coop before and after using the water fount in the coop and compare it to the outside humidity? This is my first winter with laying hens in a coop, and humidity is a concern for me. At present, I'm looking at an outside temp of -11F and a relative humidity of 64%. I have not noticed any significant difference between my coop and outside humidity. But if I did, I guess my best option would be to add more ventilation.
I don't have a covered run, and currently we have about one foot of snow on the ground. My girls won't even go outside on the snow. So putting the water outside is a non starter for me.