[TimJFowler]

Does the door threshold have one of those half-circle gaskets that pushes against the bottom edge of the door? If not you could replace the threshold with a gasketed version. I've also used a "sweep" gasket that attaches to the door and brushes against the floor. Both of those methods work pretty well.

A well weather-stripped door can make a HUGE difference in air loss, heating bills and comfort. I've got three exterior doors that are each sealed in different ways depending on the door and how well it fits in the door frame.