This paper describes a simplified heat-transfer model that defines the R values of reflective foil insulation systems in terms of the controlling convection and radiation parameters. Measured R values of foil insulation systems installed in studded test cells of ASTM C-236 and C-177 apparatus are presented; the variables included are direction of heat flow and foil number. The comparisons of the thermal model results with the experimental data presented and earlier ASHRAE correlations are good in all but a few instances. Interpretations of foil system R values in the installed condition are presented.