A three-layer model comprising two surface layers and a core is developed to explain the effect of specimen thickness on the thermal properties of thick insulation. The model explains the effect of boundary emittance and specimen density on the measurements. A relationship between the apparent thermal resistivity and the increase in resistance per unit thickness is derived. Equations are given for the densities at which the maximum thermal resistance of the core of the specimen and whole specimen occurs and for the variation of thermal conductivities of the core and the specimen with the ratio of density to density at which the minimum conductivity occurs. The densities at which the extremes occur are almost independent of specimen thickness. A V-shape and not the customary nonsymmetrical U-shape is suggested for the variation of thermal conductivity. Equations for use in curve fitting that introduce curvature at the apex and shift the position of the minimum are suggested.