An apparatus (20, 220) for inflating an inflatable vehicle occupant protection device comprises a first container (22, 222) defining a first chamber (24, 224) for inflation fluid. The first container (22, 222) has an opening (26, 226) through which inflation fluid may flow to inflate the protection device. A rupturable closure (28, 228) extends across the opening (26, 226) in the first container (22, 222) to block fluid flow through the opening. A second container (68, 268) defines a second chamber (70, 270). A plate (82, 282) is located between the first chamber (24, 224) and the second chamber (70, 270) and has a passage (84, 84a, 284 or 284a) which allows fluid communication between the first and second chambers. A rupturable membrane (86, 286) extends across the passage (84, 84a, 284, or 284a) in the plate (82, 282) to restrict fluid communication between the first chamber (24, 224) and the second chamber (70, 270) and has an orifice (100, 300) to allow limited fluid communication between the first and second chambers. Ignitable material (80, 280) is located in the second chamber (70, 270) and produces combustion products (P), upon ignition, which increase pressure in the second chamber to a predetermined pressure to rupture the membrane (86, 286) and open the passage (84, 84a, 284, 284a) in the plate (82, 282) for the combustion products to flow into the first chamber (24, 224) and effect heating of inflation fluid (F).