First, have you checked your hygrometer (salt method) to see how accurate it is. Many are badly off.
Supposing that your hygrometer IS accurate though, I would agree that 85% is really pushing your luck on the humidity and merits trying to do something about.
The most obvious thing to do is open more ventilation. Yeah yeah, I know, it will get colder in there. 'Sokay. Chickens typically do COLD just fine, it is sorta-cold-and-real-humid that gives you problems like frostbite. 30 is still *warm* for chickens. Windows don't generally make very effective cold-weather ventilation though, as they're more apt to cause draft on the chickens and also they let out less-humid air. See my ventilation page and cold coop page (links in .sig below) for more info and suggestions.
If your issue is with water freezing, then address that separately, don't be thinking you should keep the coop above freezing just to keep the water liquid (bad idea). Bring out fresh water daily or several times a day, or get a heated waterer or waterer base.
Also, can you manage sanitation to decrease moisture in the litter. A droppings board under the roost, cleaned every morning (takes literally like 10 seconds), will remove a considerable amount of ammonia and moisture from the coop right then and there. Being more proactive with the bedding cleaning (and not trying to do composting-style deep litter, if you are already having humidity problems) will also help. And of course make real sure you are not having waterer spills/leaks nor rain leaking in nor moisture rising from the ground if it's an earthen-floored coop.
Good luck, have fun,
Pat