A hollow core door is lightweight and inexpensive, but it does have a downside: it allows sound to pass through easily and is impossible to insulate internally. The term "hollow" is a misnomer. The inside of the hollow core door is actually filled with a honeycomb or baffle of cardboard that adds rigidity to the thin veneers of the door.
This cardboard baffle keeps foam insulation from infiltrating or spreading more than a few inches, making it impossible to easily or cheaply insulate the door. Here are some insulating solutions.
Replace with Solid Core Door
Replace your hollow core door with a solid core door. Even a new solid core door is not as expensive as four or five cans of insulating foam and the time and effort you would spend insulating it.
Add Foam Insulation Sheet
Glue a 1 to 2 inch sheet of foam insulation to the back of the door. This works well if you're insulating an attic or storage space where the foam door won't be seen. If the door is in a visible area, cover it with cork squares or fabric for a decorative touch.
Hang Insulated Blanket
Hang an insulated blanket or curtain on the back of the door. This is an excellent option for apartment dwellers.