Time rate of heat flow through a unit area and unit thickness of a homogeneous material under steady conditions when a unit temperature gradient is maintained in the direction perpendicular to area, expressed in Btu per hour per square foot of surface, per foot or inch of thickness, per degree Fahrenheit temperature difference across the thickness. Materials are considered homogeneous when the value of the thermal conductivity is not affected by variation in thickness or the size of the sample within the range normally used in construction.