Gauging the Impact of Exterior Shading using the Projection Factor
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).
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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 courtesy of Daniel Overbey. |
SHGC multipliers
Based on the projection factor of a permanent overhang/shading device over a glazing assembly, ANSI/ASHRAE/IES Standard 90.1 offers a series of SHGC multipliers (see Table 1).
Example: If a south-oriented curtain wall assembly has a SHGC of 0.40 and the exterior shading achieves a PF of 0.45, then a multiplier of 0.67 may be applied to the assembly:
SHGC-0.40 x 0.67 multiplier = SHGC-0.268
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Table 1: SHGC Multipliers for Permanent Projections. Table adapted by Daniel Overbey. |
A few qualifiers about the SHGC multipliers
It is important to understand a few aspects of applying the projection factor:
- As presented in the P-I version of ANSI/ASHRAE/IES Standard 90.1, the projection factor multipliers pertain to the northern hemisphere.
- The SHGC multipliers pertain to defined projection factors ranges which indicate a degree of imprecision with the multipliers.
- The non-north SHGC multipliers account for a wide wide range of Sun angles, again indicative of the imprecision of the multipliers.
- A multiplier should apply to the fenestration assembly SHGC since the ASHRAE definition of fenestration accounts for framing.
- Do not double-dip in a building energy modeling program. If you are utilizing an advanced performance tool that accounts for modeled shading, it would be inappropriate to also apply the SHGC multiplier.
North-orientation SHGC multipliers omitted in ANSI/ASHRAE/IES Standard 90.1-2016
ANSI/ASHRAE/IES Standard 90.1 no longer acknowledges SHGC multipliers for north-oriented fenestration. This represents an increased stringency in the 2016 edition of the standard. Even though at certain latitudes some true north-oriented fenestration would be exposed to direct solar radiation for short periods of time during the cooling season, the value of exterior sunshades as a practical energy conservation measure is marginal on north-oriented fenestration.
Current definition of PF does not account for vertical fins on east/west fenestration
The current definition of projection factor is simplistic and only accounts for horizontal shading. Vertical fins on the east and west-oriented fenestration is not accounted for by the current definition of the projection factor. This is unfortunate and perhaps a future edition of standard 90.1 may define the projection factor differently for east- and west-oriented fenestration.
