An airbag includes an outer shell defining an interior of the airbag, and a divider positioned in the interior so as to divide the interior into a first chamber and a second chamber. The divider includes a body portion and at least one flap attached to the body portion along an edge. The divider is attached to the outer shell so as to form a gas-tight seal between the outer shell and the body portion and so as to form a gas flow passage between the at least one flap and the outer shell.