Always install it with the door closed. The bottom of the door is also a place where you can lose a lot of heat. The solution is also weather stripping. You can use a door sweep on the face of the door, and it can be attached with screws or nails. Windows and Electrical Outlets Draw in Cold Windows are also a place where you can lose a lot of heat or feel drafts coming through.
Cold air can leak in at the bottom of the windows, and at the top where the two windows meet.
To seal the bottom of the window right here, use weather stripping, which has a pressure-sensitive backing that can be peeled right off and stuck on the bottom of the window. You might also want to use caulking to seal the outside of the window where the window frame right comes up against the house. Take a caulking tube, run a bead of it down and seal up any gaps. It can also be used around door frames. Lastly, electric receptacles in exterior walls can be a source of real heat loss.