I have a full set of stairs that walk up into an unfinished attic space. The full sized access door to these stairs is in a hallway as if its a closet. Unfortunately, the max distance I have under each stair before I hit the attic-side of an interior wall (solid blue line in picture below) is only 6 inches. What is the best way to insulate this under the stairs so heat isn't lost from the interior closet pictured? I've drawn a diagram below:
Best way is to add a door at the top to stop airflow and add insulation to that door. On the other side - insulate the door leading up to the attic. There are cheap door insulation kits sold for doors leading to garages... same thing here.
If you don't go the route of stopping drafts I would insulate door and add padding and carpet to stairs. Stairs notoriously let in air. Some cheap carpet would not only insulate but be an air stop.