garage condensation problem

Greetings all, am looking for some advice re. severe condensation in my garage,which is 20 by 10 foot wooden with felt roof. (there is no rain getting in anywhere). Car,which is only one year old, is sometimes absolutely soaked with condensation which won`t do it any good!
Strangely,one side of car is more affected than the other,and this side of garage is about eight feet away from a tree and high hedge,so don`t know if there`s a clue there.Have fitted extra vents,three at each end of garage,but made no difference!
Have fitted a de-humidifier,which eventually clears condensation,but is expensive to keep running 24/7. Has anyone else had this problem and managed to solve? Would some bathroom type extractor fans be likely to help,these are low wattage,so much cheaper to leave running. Have tried Googling for info,but only got ads. for heaters etc.!
Sorry for long post,but would really appreciate any info. on this. Regards to all.........Doug.
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