A cabinet having an aperture passing through a surface thereof. A fan is mounted over the aperture in the cabinet. The fan is adapted to produce a flow of air through the interior of the cabinet when operating properly to exhaust such flow of air externally of the cabinet. A resilient membrane is mounted to said surface of said cabinet. The resilient membrane has formed therein: a plurality of slits extending radially from a center of a region of the membrane. The slits are spaced about the periphery of the aperture to form a plurality of flaps in the membrane. The flaps are configured to pivot from a nominal position outwardly from the surface of the cabinet from the flow of air produced by the fan when the fan is operating properly and to return to the nominal position when the fan is operating improperly to inhibit air externally of the cabinet from entering into the interior. The fans have a hub and a grate. The membrane has an aperture formed therein. The aperture is at the center of the region. The aperture in the membrane is disposed over the hub and the flaps are disposed over the grate when the fan is operating improperly. With such an arrangement, the mass momentum of the air causes the flap (i.e., a check valve) material to deflect outward and open a path for the air to escape. The air can continue to exhaust from the fan while the fan is operating; but, if the fan fails and stops operating, the flap will collapse and seal over the aperture thereby inhibiting air leakage into the cabinet. Further, the flap also serves as a visual indicator, by its "closed" position, to indicate whether or not the fan is operating.