Like other have mentioned, ventilation is most important. The birds themselves create humidity from their breath, in addition to any other moisture source in the coop. You want the air in the coop recycled with fresh air constantly.
Other suggestions.
Fix any leaks in the coop where rain or melting snow may enter.
Keep a vented air space under the coop, so moisture from the ground doesn't wick up into the coop floor. A heated coop can add to this problem in seriously wet climates.
Do a complete clean out of all the old floor litter, which may hold moisture, from droppings, spilled water etc. A deep, fresh layer of shavings may help.
Does your waterer have a catch basin under it, to prevent overflow, or spills from mixing with the litter?
A dropping board under the roost, makes for easy clean up of night droppings, which should be done daily.
If the coop is actually heated to much, it can cause evaporation of water from damp litter, and the waterer, which will condense on the walls and roof. Unless you have chicks, a mature chicken shouldn't need heat, unless it's extremely cold. You should have a method of keeping the water from freezing though.
Sometimes the air humidity is extremely high, just from several days of wet weather. Hard to counteract this problem. Air flow is about the only way to prevent this, so again it falls back to good ventilation.
Good luck!
