U-values gauge how well a building element allows heat to pass through. The lower the U-value, the greater a building element's resistance to heat flow and the better its insulating value. The U-value is the reciprocal of the total R-value, inclusive of external and internal surface resistance. Note: neither a material nor a material layer has a U-value. As U-values encapsulate surface resistance, they only make sense for building elements. With the building element "window" the U-value is the weighted mean of the U-value of the glazing unit, the frame, and the linear thermal transmittance of the glazing unit's edge spacer, multiplied by the length of the glazing unit's perimeter.