What is freezing is most likely the weatherstipping that surrounds the doors.
Get a can of silicone spray from the auto parts store. Put on a pair of exam gloves, which are great to keep around the house when doing messy chores. Spray the silicone liberally on a rag, and wipe it on the weatherstripping until it is coated evenly with the silicone.
Keep repeating the spray/wipe process until you have treated all of the weatherstripping. This should resolve your problem, but I would suggest that you do this each year before the winter weather sets in.