Answers

Ventalation is one of the most important parts of your roofing system.  Has proper intake air flow coming in from the eaves of your home, where your gutters are located, and exausting through the ridge of your home.  All roofing manufactors require this as part of there warranty information because it does not only affect the condisation in your attic space but will also weaken your shingles and lower there live span.  If your home already has proper insolation and proper ventatation another option would be to add a power fan unit on your roof near the ridge with a humidistat built it.  This will turn on automatically at whatever tempature you set and also when moisture builds but will turn on.  Moisture in the attic space is common in winter months due to cold tempatures outside and heat from your attic space meeting.  I would have a roofing company come out and inspect your soffits to make sure they are ventalated and also to ensure you have a ridge vent installed on your home to start.