I was looking on the Internet for some Window Wells to prevent dirt
from piling up near my crawl space vents. Then I see something like
this on the Internet suggesting that the thinking is now to seal the
vents to save on energy and keep out moisture?

http://www.basementsystems.com/crawl...space_vent.php

What is recommended, to seal the crawl space vents or leave them open
for air flow?