The term U-Value is often confused with thermal conductivity. Both are measured in Watts per meter-Kelvin (W/mK). Thermal conductivity refers to how well a single insulation product transmits heat. U-Value refers to how well a build-up of materials transmits heat.  Multifoil roof insulation rolls have very low U-Values. SuperFOIL SF19BB has a U-Value as low as 0.010 W/mK.