Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Measuring glazing performance: key concepts

Glazing performance is typically measured according to U-value, R-value and solar heat gain coefficient.

On this page:

  • U-value or R-value
  • Solar heat gain coefficient (SHGC)
  • shading coefficient (SC)

Though some glazing systems have additional features that are descibed using other parameters (see BRANZ Bulletin 471 and the Window Energy Rating System, R-value, U-value and SHGC are the most commmonly used measures of performance.

U-value and R-value

U-value

The U-value, or heat transfer coefficient, is a measure of the quantity of heat that will flow through a specific section of material. It is expressed as W/m2 0C. The lower the U-value, the smaller the heat flow, so a lower U-value indicates better insulating properties of the material.

R-value

The R-value is the thermal resistance or insulating value of a material. The R-value is the reciprocal of the U-value, so a higher R-value indicates better thermal insulation properties of the material.

As the U-value and R-value are reciprocals of each another, one can be calculated from the other – U = 1/R and R = 1/U.

Total product performance

The thermal performance of a window is influenced by both the glazing and the frame. In order to accurately determine the thermal performance, both components must be taken into account. (The air infiltration through a window system is assumed as a constant, so does not influence the R-value in testing or modelling.)

The thermal performance (U- or R-value) of a glazing system without the frame is typically referred to as the centre of glazing U or R-value(Ucog or Rcog), the frame as Uframeg or Rframe while the thermal performance of a total product including the glazing system and the frame is referred to as Utotal or Rtotal.

For a large window, the U-value or R-value at the centre of the glazing (Ucog and Rcog) will have a greater impact on the overall thermal performance than for a small window, which will be affected more by the frame performance (Uframe and Rframe) and less by the performance of the glazing system.

Solar heat gain coefficient (SHGC)

The solar heat gain coefficient (SHGC) is the total fraction of available solar radiation that is transmitted through the window as heat gain. It is expressed as a value between 0 and 1, which gives the proportion of energy from the sun that passes through the window (including frame) into an interior space.

For example, a SHGC value of 0.6 means that 60% of the solar radiation will pass through the window to the interior and 40% will be excluded.

Windows for passive solar heating must have a high SHGC so the solar radiation is able to enter through the glazing system. Conversely, windows designed primarily for passive solar cooling must have a low SHGC, as the glazing must exclude a large amount of the solar radiation.

Shading coefficient

The shading coefficient (SC) is an older means of assessing the total amount of solar radiation passing through a glazing system (not including the frame) and is not commonly used now and has been replaced in literature by the SHGC. It is derived by comparing the solar radiant heat transmission properties of any glazing system against the solar radiant heat transmission properties of 3 mm clear glass, which has an SHGC of 0.86 (86%) and is given a shading coefficient of 1.0.

The shading coefficient is calculated by the equation SC = SHGC / 0.86.

For example, for glazing with a SHGC of 0.6, we have 0.6 / 0.86 = 0.69, so the shading coefficient (SC) is 0.69.