Cold roof & warm wet air = condensation.
When the warm air touches the cold surface it can no longer contain the same volume of water - it condenses out on the cold surface.
Putting a warm car to bed may help cause the problem - best to let it cool before putting away. Similarly steamy or just warm air from the house may be a factor.
Ventilate the garage as much as possible - even in wet weather - keep warm air out of there and put the car away cold.
Items already in the garage may also be carrying moisture that you can't see.
Garden centres sell cheap humidity meters (£5) that can show you what's going on.
If you really need to keep the garage warm then you might consider insulating the interior to keep the internal air away from the cold surfaces.
Peccavi
November 2010
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