If the humidity is 90% when you open the vents, then you need more vents. How are you measuring humidity? I think you are confusing yourself unnecessarily by the humidity. Forget it.
Try to picture air flow. I know you can't see air, so plug in your imagination. Warm air is light and it wants to rise, so you need vents up high for the warm moist air to escape.
Picture cold air now. It's heavy, much heavier than warm air. So it pushes in through the pop holes down low and exerts pressure on the warm air inside. This causes a natural air flow pattern from down low to up high, and air escapes through vents at the top. You need to make sure there are large enough openings up high for the warm moist air to escape. This is air flow, not drafts, which are bad.
Now, go back and read my first post and consider adding a little amount of heat to counteract the freezing temps. You only need to raise the temp to 33F not 75F. Wrapping coop and run in plastic will hold in the moisture and risk of frostbite becomes higher.