The solar heat gain coefficient (SHGC) is a ratio that defines the amount of solar energy that reaches a glazing assembly (including both the glass and frame) with the amount that gets through to the inside. It represents the ability of a glazing assembly to resist heat gain from direct solar radiation.
A high SHGC indicates poor resistance. That means most of the solar radiation arriving to the window or skylight is getting inside and turning into heat. A low SHGC indicates good resistance. Meaning, only a small percentage of the Sun’s rays makes it through the window or skylight.
When exterior shading devices are present on a building, the degree to which the devices protect the glazing assembly should be factored into considerations of energy performance. ASHRAE has defined a simplified method of accounting for a shading device’s impact on the performance of a glazing assembly by reducing (increasing the resistance of) the glazing assembly SHGC based on the effectiveness of the exterior shading device. The effectiveness has been defined as the projection factor.
The ASHRAE definition of projection factor, as offered in ANSI/ASHRAE/IES Standard 90.1-2016, is as follows:
projection factor (PF): the ratio of the horizontal depth of the external shading projection divided by the sum of the height of the fenestration and the distance fro the top of the fenestration to the bottom of the farthest point of the eternal shading projection, in consistent units.
It is also important to acknowledge how ASHRAE defines fenestration:
fenestration: all areas (including the frames) in the building envelope that let in light, including windows, plastic panels, clerestories, roof monitors, skylights, doors that are more than one-half glass, and glass block walls.
Stated differently, the projection factor is a simplified method to account for the performance improvement of fenestration that is protected from direct solar radiation by exterior horizontal shading. The projection factor is based on the ratio of an overhang/shading device depth (A) to the overhang/shading device height above the glazing assembly sill (B) (see Figure 1).
|Figure 1: The projection factor (PF) is based on the ratio of the overhang/shading device depth (A) to the overhang/shading device height above the glazing assembly sill (B). Illustration by Daniel Overbey.