GB beat me to it, weatherstripping, and a good door sweep.
you can cut a small triangular notch out of the bottom of the doorsweep to let the cord runthough, and its a great idea to strap the cord to the door on either side to keep it from bunching up when you open or close the door, and if you place the cord as close to the frame side of the door (not the jam side with the knob) as you can, it will barely move at all when you open and shut the door. this same tech works great for sealing a room for co2, ive done it in the past for a couple of closet setups,, the only mod i made was to use a tiny amount of greatstuff (minimal expanding foam sealant) to actually seal around where the cord went through the sweep on the bottom of the door