It's not that important to get all concerned about humidity. It's air flow that's the key to not having that moisture condense on surfaces and freeze. As long as air flow moves the moisture in the air through the coop and out the vents, it won't condense and settle on the combs and freeze.
It's snowing on my coops right now, and even without a means to measure humidity, I can be pretty certain it's 100%. It's also 2F. The pop holes in my coops are left open as are the vents at ceiling level. This allows the air in the coops to be constantly in motion so no moisture is allowed to condense on surfaces, including chickens. Therefore, everything is frozen outside except my chickens' combs.