My experience with covers made of Kimberly Clark Evolution material is that they are not truly waterproof. They are a triple layer cloth material that is extremely water repellent. It is breathable so condensation is not trapped underneath it. If a cover is completely 100% waterproof it was trap condensation.
I had a Covercraft cover made of Evolution on my Seven that was caught in an overnight rain storm. The next morning and there was a small lake on top of the cover (I didn't have the top on the car) over the passenger area. Underneath the car was completely dry, the cover had absorbed some moisture and needed to be dried out (took about an hour in the sun).
I now use covers of Evolution on my Elans and have had similar experience. Thumbs up!
Kiyoshi