Cooled infrared detective assemblies are usually packaged in a high-vacuum dewar which provides good thermal insulation and reduces cooling power input requirements. This paper describes the development and evaluation of a nonevacuated infrared detector package which may be an attractive alternate in some system. applications. The nonevacuated package is insulated with polyurethane foam and where mounting space permits can often directly replace the more expensive conventional vacuum package. The paper also describes a generic thermal mathematical model which has been developed as a design tool for these non-evacuated packages.