Duck or goose down, mostly a byproduct of birds raised for meat, gets a water-repellent finish before it reaches the sewing floor. Soaked down can insulate worse than no bag at all.

Light synthetic fabric is cut into the outer shell and inner lining, plus strips of mesh for the internal walls.

Mesh walls are stitched between shell and liner to make box-shaped chambers, often offset so each one is a trapezoid. Cheaper and lighter bags sew shell straight to liner, which leaves cold lines at the seams.

A long zipper runs down one side. Mummy-shaped bags usually stop it short of the feet, since every zipper leaks heat.

Compressed air pushes down through a metal tube into a small opening left in each chamber. Computer controls release a weighed charge per chamber, with more around the torso and feet than elsewhere.

A skilled machinist stitches every insertion hole shut by hand on a sewing machine.