A closure structure for the relief aperture of an airbag cushion inflator includes a closure core having outer and inner surfaces that is disposable into the relief aperture from the exterior of the inflator to fill the relief aperture. A sealing flange continuously secured to the periphery of the outer surface of the closure core abuts the outer surface of the inflator. A temperature-sensitive venting window formed in the closure core precludes the passage of pressurized inflation gas during a predetermined first ambient temperature condition and opens to emit pressurized inflation gas during a second hotter ambient temperature condition. The venting window includes a venting passageway extending between the inner and the outer surfaces the closure core sealingly traversed by a relatively thin venting membrane made of a temperature-sensitive material exhibiting less structural robustness at the second ambient temperature condition than at the first ambient temperature condition.