Thermal performance of windows and doors is a complex calculation based on the frame, sash, panel, and glazing performance of each unit. In addition, wide disparity in thermal performance is likely to exist among units due to size and configuration.

Shading Coefficient (SC COG)

The ratio of the total solar heat gain through a particular glass compared to the total solar heat gain through 3mm clear float glass. (86%) The shading coefficient of 3mm clear float glass is by definition 1.0 and represents a base glass performance. The lower the shading coefficient, the less heat gain and consequently the more shading is provided by the glass.

The shading coefficient is calculated as SC = SHGC/0.86

In areas such as Northland you would expect to see a higher use of tinted glass, which reduces the heat gain particularly on western and northern glazed aspects. In the South Island you would expect to see a lower use of tinted glass to allow greater heat gain from the sun in a cooler climate.

R Value

The R value is the value of thermal resistance of a building element. It is the sum of the surface resistances on each side plus each component of a building element. It is the inverse of the U value – R=1/U and is expressed as m2 ºC/W.


RWindow is the R value measure as measured within a window frame. Different frame types impact on overall thermal performance in different ways. In cooler climates or areas where heating is required more frequently, having a higher R window value provides greater insulation protection.