All our outdoor Custom Car Cover materials are breathable – this allows moisture which is trapped under the car to escape through the cover, preventing a greenhouse condition of condensation, mold, mildew etc. If a car cover is waterproof, than any condensation and moisture will be trapped under the cover and cause mold, mildew & bacteria, oxidation and rust. This can be easily avoided by covering your car with a breathable material, thats weather resistant. Beware of covers that claim to be 100% waterproof as a waterproof car cover used for long term will damage your vehicle. A car cover should be water and weather resistant, to keep the vehicle from getting wet from dew or moisture, but not waterproof (or rainproof).
See here for custom fit, breathable car covers by: Coverking and Covercraft