A floor grommet for use in building and office structures supplied with air conditioning via under floor plenum. Directed flow of conditioned air is optimized by limiting the escape of air from the plenum into the space above the floor by leakage through floor openings provided for power cables, data cables and the like. Specialized floor grommets installed in the cable openings are comprised of a surrounding frame mounting sealing elements comprised of thin, flexible elements which are anchored at one end in the grommet frame and extend toward the center of the opening, from each side, to effectively close the opening against significant flow of conditioned air from the plenum below. Cables passing through the grommet opening cause minimal deflection of the flexible elements to limit the escape of conditioned air. Preferably, the resilient, flexible elements are filamentary in nature. To particular advantage, the filamentary sealing elements may be installed in multiple stages, with stages underneath being in contact with stages above, in outer portions thereof. Additionally, solid, van-like sealing elements may be provided to assist in centering of the cable elements, when cables are installed, and to provide a substantially complete seal, for those openings in which no cables are installed. The floor grommet of the invention is designed for initial or retrofit installation. Significant economies in the usage of conditioned air are realized when the floor grommets are employed.