An improved floor configuration for the interior rear portion of a motor vehicle wherein at least one region, made of a fracture-prone material such as plastic and typically the spare tire cavity or foot recess cavity region, is covered by a sealing cover assembly having properties of high ultimate elongation, fluid impermeability (both liquid and gas impermeable), and good crack or tear-resistance. In one embodiment, the sealing member comprises a carpet having a flexible, stretchable backing layer, preferably a plastic sheet having high ultimate elongation, high yield strength and good resistance to hydrocarbons. In the event of a collision-induced fracture or crack in the floor, the sealing cover assembly prevents any direct air communication between the outside ambient atmosphere and the interior passenger space of the vehicle so that flammable gases or noxious fuel vapors originating from a nearby damaged fuel tank or from the deformed engine compartment of a colliding vehicle cannot enter the interior passenger space of the vehicle. The elasticity and stretch features of the carpet with seal layer ensure that any sharp fracture edges resulting from floor deformation remain unexposed so that the passengers are protected against injury. In another embodiment the high ultimate elongation properties of the carpet seal combination are enhanced by stretchable folds provided in those regions adjacent the footwell side walls of the plastic cavity shell.