Your basement or crawlspace should not be sealed shut. The area needs ventilation, even if you do not use it often. If your basement or crawlspace is damp, do not close or block the vents - doing so may cause mold and lead to rotting. The mold can spread into living areas of your home.
Sometimes you cannot see moisture in your basement but you know it is there. The air feels wet and smells moldy. Here's what you can do to find out if there is moisture permeating through the dirt floor of an unfinished basement:
Place clear plastic on the dirt floor for a few days to see if water collects on its underside. If you have moisture and ventilation problems, it is best to contact an HVAC or ventilation specialist. Once your basement is dry, you can close foundation vents in the winter to save energy. Open them when it warms to allow moisture from the house to escape.