Crawl space vent covers have manual or automatic dampers that can be adjusted to keep cold air out from under the house during the winter season. The vent also has an outer grille and a finely meshed screen that keeps insects and other animals from entering.
The manual vent has a lever, usually located on the front, which the user can push up or down to manipulate the damper. The automatic damper is temperature-controlled. The vanes open fully when the temperature hits 70 degrees Fahrenheit and close at 40 degrees Fahrenheit. The vanes adjust for temperatures within that range; no power source is needed.