Foundation/crawlspace vents have covers that really should be removed entirely for proper ventilation.  In fact, I strongly advocate against installing vent covers at all.  Ventilation is absolutely critical to maintaining proper humidity levels in the crawl space under the home and to prevent moisture and insect damage.  

I recommend keeping crawl space vents open all year, provided the water supply pipes in the crawl space are properly insulated.  For more information on foundation/crawlspace vents please check out this YouTube video I made several years ago on the topic: