A clever owner has come up with a solution to close off the sides of the doors at the framing where not only cold comes in, but so do critters.
Foam weather stripping with peel off backing can be attached to the bottom of the door, with an additional a few small nails recommended to ensure it stays in place. Some doors have larger openings than others.
With a little creativity, you can add an additional layer or more by sticking the pieces together, and tapering the bottom to fit the shape of your door. Easy. Affordable. Smart!