Although it may seam more difficult to close your foundation vent with a concrete block it is actually not that hard and far more cost effective. The franchise companies sell their “vent covers” for around $25 each. To close your vent with a cement block will cost you about $3. Vent covers will not keep out the winter temperatures or keep the mice out (I have see them chew through metal). Cement block are easily accessible at you local home improvement center as well as the mortar or cement...

Homes built on a crawl space needs ventilation! Right? Right! Notice I did not say “Needs vents”, that’s because a crawl space does not need the foundation vents (that go outdoors) if, and I quote:IRC 2006 and 2009, Section 408.3Unvented crawlspace Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:Continuously operated mechanical exhaust ventilation at a rate equal to 1.0 cfm (0.47 L/s) for each 50 ft² (4.7 m²) of crawlspace floor...