Passive Design

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

Measuring glazing performance: key concepts

Measuring glazing performance: key concepts

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

On this page:

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

Some glazing systems have additional features that are described using other parameters (see BRANZ Bulletin 579).

R-value

The R-value is the thermal resistance or insulating value of a material. The higher the R-value, the better the thermal insulating properties of a material.

(New Zealand uses metric R-values. You may see reference to imperial R-value measures used in some overseas countries. There is a huge difference metric R 3.0 is imperial R 17.0! To convert from imperial to metric, divide by 5.678.)

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 (R-value) of a glazing system without the frame is typically referred to as the centre of glazing R-value (Rcog), the frame as Rframe while the thermal performance of a total window including the glazing system and the frame is referred to as RW.

For a large window, the R-value at the centre of the glazing (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 (Rframe)

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.

 

Updated: 04 July 2017