In the summer months, your foundation vents allow air into the crawlspace under your house, and this circulation is essential for keeping moisture and mold at bay. This function isn't as important in the winter, though, and the cold air coming in through the vents can be an energy drain. You can always insulate the floor to keep your feet warm -- but blocking the vents also does that while protecting your pipes from freezing, and it's easier to do. Premade foundation vent covers are inexpensive, but if you can't find any that fit, you can always make your own.
Things You'll Need
Rigid foam insulation
Measure the dimensions of one of the vents you want to cover, using a tape measure. If the vent is divided in the middle, measure the dimension of each half of the opening.
Transfer the measurements onto a sheet of rigid foam insulation, and cut out the insulation with a drywall saw. Test it by putting it in the opening. It should fit tightly and be slightly difficult to remove.
Cut a rectangle from a sheet of 1/2-inch construction-grade plywood that's about 2 inches wider and longer than the foam rectangle, using a circular saw. Center the foam on the plywood, and glue it with construction adhesive.
Prime the outside of the plywood with wood primer. After the primer dries, paint the plywood with two coats of paint in a color that matches or blends with the foundation color. Fit the cover into the vent opening at the beginning of winter, and remove it in the spring.
The crawlspace vents aren't always easy to locate; they are sometimes hidden by bushes or tall grass. They're usually about 12 inches above the ground, and there should be at least one on each side of the house.
Secure the cover with one or two screws if it doesn't stay by itself.
Don't leave the vents covered during the summer. In hot weather, humidity and condensation can quickly rot the framing under your house.