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High humidity in finished garage

I live in southeastern Illinois. Last summer (2013) I built a finished room in nearly 1/2 of our finished 2 car garage. Two layers of heavy plastic were placed on the garage concrete floor for the new room. The room floor has 2x6 floor joists with 6" fiberglass batt insulation. All existing garage walls were already drywall/painted finish with fiberglass batt insulation. The two new walls of the room are 2x4 framed, drywall/painted finish with fiberglass insulation and paper vapor barrier on the interior of the walls. The room/garage attic is insulated with 2x6 fiberglass batts. The room/house humidity runs in the 40/45% range. This new room is cooled/heated with a Fujitsu mini split unit which cools and heats the room well. NOW THE REAL PROBLEM......WINTER. The unheated remaining portion garage has been in the 40 to 55 degree range and humidity is in the 75 to 90% range. When outside temps drop below 30 degrees the garage humidity becomes noticeable but really noticeable at outside temps of 10 degrees down to below zero. Moisture forms on the steel insulated overhead door as well as the entry door and freezes on them at the colder temps. Water droplets also form at the ceiling/wall (south wall) intersection and will even run down the wall. The attic insulation and roof sheeting all seems to be dry, but some mold has began to form on the edge of the osb sheeting I have on the attic floor where it meets the edge of the attic ladder (which has an insulation blanket on it). Also, some of the drywall taping is pulling loose from the ceiling/wall joint. The attic has a ridge vent and soffit vents with the foam baffles at the eaves which are not covered up. I need suggestions.

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