Originally Posted by 
JDNSW
				
			 
			I'm afraid you are not strictly correct. They do not allow outward venting, except, as you point out, to relieve excess pressure when hot. But they must allow inward venting, as otherwise, by the time the tank was empty, it would have a pretty good vacuum in it, and fuel delivery would be difficult!
The air that is drawn in will contain water vapour, which will be absorbed by the ethanol. If the tank is only partly filled, cool temperatures overnight will mean more air will be drawn in, which will then expend during the day, probably requiring pressure relief, and the cycle repeated next night, with fresh, moist air.
The only way to avoid this is to keep the tank completely full.
As I pointed out above, if there is no ethanol in the fuel, if the air is moist enough, and there is a sufficient temperature cycle, the water vapour in the air will condense on the cold top of the tank, from where it will run to the bottom of the tank. Since it is immiscible with and denser than petrol, it will form separate droplets in the bottom of the tank, that cannot progress further into the fuel system, and hence are largely harmless, although owners of Series vehicles with rusted out fuel tanks or rust pinholes in the suction line may not agree!