It depends on the cover - get one with a thermol base that will not only pretect the van from the sun but also hail etc. A little more money but a great investment. Try Stormcover's.

Beware of Stormcovers. I bought one for $299 in Sept 09. When I went down to check on my van today the cover has split from front to back on both sides, along the top corners. The material has also let go along the side of the zips. The material on the roof feels like tissue paper.
The cover had a one year warranty, replace or repair. Brian.