Liquid helium is stored in containers fitted with 7-cm-thick "superinsulation" consisting of a large number of layers of very thin aluminized Mylar sheets. The rate of evaporation of liquid in a 200-L container is about 0.7 L per day. Assume the container is spherical and that the external temperature is 20oC. The specific gravity of liquid helium is 0.125 and the latent heat of vaporization is 21kJ/kg. Estimate the thermal conductivity of superinsulation.