Wiggy's bags can be purchased vacuum packed and stored that way for years without loss of loft; just give them a shake and over 90% loft is restored within seconds. I'd imagine most synthetic bags can be stored in a compression sack for years so long as they're kept dry. I'm not a fan of down due to the fact that it's worthless when wet, but if you have a down bag and have the room to keep in packed loosely it should stay dry in a car.