Gas-cooled reactor systems can benefit from the use of internal metallic-foil insulations which take advantage of the relatively low thermal conductivity of the coolant gas itself. Tests have shown that, for design purposes, Nusselt, Grashof, and Prandtl number correlations for vertical gas spaces form a good basis for finding optimum foil spacing and for approximating insulation performance. Tests were conducted chiefly on a spirally wrapped foil arrangement in which in. spacing between adjacent foil turns was maintained by strips of corrugated sheet metal 1 in. in width. Results from this arrangement in an atmosphere of helium have shown gross effective thermal conductivity values to be approximately 150% of the values for the gas itself at pressures below 200 psia. From 200 psia to 1000 psia, conductivity increases with pressure to values approximately twice those for the gas itself. For the specific geometry tested effective conductivity was shown to be a function of mean insulation temperature, gas pressure, temperature gradient across the insulant, and insulation thickness.