A car cover should, above all, be FUNCTIONAL. If it doesn't offer the protection you want for your car, there is no point in using it.
The kind of protection your car needs depends on the storage conditions. Various storage conditions present various risks which require relevant features of the cover.
to keep it clean
non-scratchy inner lining
to protect from sharp edges on
zippers, bikes, garden tools etc.
CAR COVER FEATURES
to protect from damaging sun rays
to allow moist to escape when it evaporates
not absorbing the moist in the air
to protect it from rain and bird/treedroppings
R I S K S
Breathable and Non-hygroscopic
These two features are extremely important in any unheated environment, to be able to cope with condensation.
Condensation occurs when temperatures rise or drop quickly: on surfaces that are colder than the air itself, the humidity in the air transforms to water droplets. It happens outdoors as well as indoors in unheated spaces. When the temperature on the surface reaches the level of the temperature in the air, the droplets evaporate back into air.
If the cover isn't breathable the vapour can't escape and the car will remain wet under the cover.
If the cover absorbs moist it will lie on the car like a wet cloth, therefore the material must be non-hygroscopic.
If condensation is a serious problem in your storage, and you don't have the opportunity to let your car "air"
Read more about condensation HERE.
Even if your car is stored indoors, UV-rays may present a risk when the car is parked near a window or under a glass roof.
If your car is parked in a carport UV-rays may touch the car, depending on the shape and position of the carport.
A car cover can NOT prevent condensation from happening.
Special storage systems are designed to tackle causes of condensation, each in their own way.