Thermal insulation is provided which includes randomly distributed inorganic fibers and about 5-500 wt. % unexpanded hollow microspheres comprising a polymeric shell having disposed therein a blowing agent or gas, said unexpanded hollow microspheres increasing in size when heated. Also provided is an insulated attic, including a plurality of rafters, an attic floor and a thermal insulation comprising randomly distributed glass fibers in at least about 5 wt. % hollow expanded microspheres having a diameter of 20-140 microns, which have been expanded from about 6-40 microns. The hollow expanded microspheres reduce the thermal conductivity of the thermal insulation by at least about 5%, and have shown improvement up to 19.4% decreased thermal conductivity during tests at loadings from 30% to 257%. Additionally, calcium acetate and cupric carbonate additions to glass fiber insulation products showed improvement by at least about a 1% decrease in thermal conductivity.