In cold climates and with rising energy prices, insulation cost plays an important role in total building cost. Fossil fuel prices may, as well, play a similar important role, which makes insulation economy and optimization an important goal. In this paper, the authors develop a more consistent and more general mathematical model. An interactive program to select the best insulation thickness and to determine easily the many calculations for various suggested and available systems has been also introduced. The program is written in FORTRAN 77. A flow chart is displayed to assist the reader in programming it on his own personal computer (see Fig. Al). Also, some important runs have been given to help in designing such pre-suggested parameters.