Putty-like gap-filling compounds for insulation that can be utilized at temperatures from about -420.degree. F. (-250.degree. C.) up to about 500.degree. F. (260.degree. C.) are described herein. Embodiments of these compounds contain about 25-50 volume percent base material, about 50-75 volume percent microspheres, and about 0.1-0.3 volume percent catalyst. Embodiments of these compounds contain about 70-80 weight percent base material, about 20-30 weight percent microspheres, and about 0.1-0.5 weight percent catalyst.