One of the best ways to insulate a door is with a bumper weather strip. It allows for some shifting of the building and yet still providing a good seal against drafts coming through the door opening. It is usually a metal rail with a vinyl bead that is somewhat soft and can compress when an object pushes against it.
They come in wood and foam which was the original old-school grandpa style which is still available, the aluminum one comes in a metal rail with adjustable slots so when it gets screwed to the jam when it shoved against the door it may be adjusted. They are attached to the jam going around the perimeter of the door so that when the door closes it compresses the bead on the weather stripping.
Most of these types of weather stripping have slots for the screw holes so after it has been installed the screws can be loosened and the molding can be made tighter or less tight up against the door. So keep in mind when drilling pilot holes for the screws make the hole in the center of the slot so the molding can be adjusted either tighter or looser against the door at a later date.
When installing the most important thing to remember is not to compress the bead too hard against the door as you are installing it, as 1 ounce of pressure every foot as you’re installing it is easy however when the door goes to close against 6 feet of the bead it tends to be hard to close the door, you may even have to use your shoulder to push it closed.
The seal created by the very light compression of the bead provides excellent draft resistance and provides a long term weather stripping for your door. This saves energy while providing indoor comfort.
They are available in 3 finishes, aluminum (mill), white and bronze/brown.