Understanding the SHGC

Solar Heat Gain Coefficient (SHGC) is a critical metric used in the building industry to determine the amount of solar radiation that enters a building through its fenestration system, including windows, doors, and skylights. It is defined as the ratio of solar heat gain through a fenestration system to the incident solar radiation. SHGC is an essential factor to consider during the design process of a building as it directly affects the amount of heat gain that can cause a building’s internal temperature to rise, thereby increasing the load on cooling systems and energy consumption.

The SHGC is determined for the overall single or multi-glazed fenestration product, including the sash and frame, in accordance with ISO-15099 by an accredited independent laboratory and labeled or certified by the manufacturer. The SHGC value ranges from 0 to 1, where 0 indicates no solar heat gain, and 1 indicates maximum solar heat gain.

However, there are some exceptions to the determination of SHGC values. According to §4.2.1.2 of the International Energy Conservation Code (IECC), the SHGC requirements for the overall fenestration area can be met by using either of the following alternate methods:

(a) Shading coefficient (SC) of the center of glass alone multiplied by 0.86 is an acceptable alternate for compliance with the SHGC requirements for the overall fenestration area. Shading coefficient is the measure of the amount of solar heat that passes through a piece of glass or other fenestration products compared to the amount of solar heat that passes through a reference piece of clear glass of the same thickness. It is also expressed as a value between 0 and 1.

(b) Solar heat gain coefficient (SHGC) of the glass alone is an acceptable alternate for compliance with the SHGC requirements for the overall fenestration product. The SHGC of the glass alone is determined by taking into account the solar radiation transmitted, absorbed, and re-emitted by the glass. It is a complex calculation that requires detailed knowledge of the optical and thermal properties of the glass.

The use of shading coefficient or SHGC of the glass alone as an alternate to determine the SHGC values for the overall fenestration product is allowed because the fenestration product’s sash and frame typically have a low SHGC value. As such, their contribution to the overall SHGC value is relatively small and can be ignored in some cases.

The Solar Heat Gain Coefficient (SHGC) is an important parameter used in building energy performance analysis that measures the amount of solar heat gain that passes through the fenestration (windows, skylights, etc.) in a building. It is an indicator of how well the fenestration performs in blocking the heat from the sun. The SHGC value is an important factor in determining the cooling load of a building.

The Energy Conservation Building Code (ECBC) 2017 mandates specific SHGC requirements for fenestration based on the location and the orientation of the building. Fenestration with permanent external projection provides permanent shading to the fenestration and reduces solar heat gain. In this blog article, we will discuss how to calculate the effective SHGC for such fenestration.

Step by step method to calculate the effective SHGC:

Step 1: Projection Factor (PF) The projection factor is the ratio of the length of the projection to the height of the fenestration. It is calculated as follows: PF = Projection length (in mm) / Fenestration height (in mm) The projection length is the distance measured horizontally from the plane of the fenestration to the outermost point of the projection. The fenestration height is the height of the fenestration measured from the base to the top.

Step 2: Fenestration in non-cardinal direction The fenestration in the non-cardinal direction refers to the fenestration that is not facing north, south, east, or west. The fenestration in non-cardinal direction factor (PC) is calculated as follows: PC = 1.0, if fenestration faces north or south PC = 0.8, if fenestration faces east or west PC = 0.6, if fenestration faces north-east, south-east, north-west, or south-west

Step 3: Maximum Allowable SHGC (SHGCmax) The maximum allowable SHGC (SHGCmax) is the SHGC requirement specified in Table 4-10 of ECBC 2017 for the specific location and orientation of the building. The SHGCmax values are different for different locations and orientations.

Step 4: Coefficients of Shading Equivalent Factors (SEF) The coefficients of shading equivalent factors (SEF) are given in Table 4-11 of ECBC 2017. The SEF values are based on the latitude of the location of the building. The SEF values are calculated as follows: For latitudes greater than or equal to 15 ºN: C3 = 0.0 C2 = 0.16 C1 = 0.66 C0 = 0.0 For latitudes less than 15 ºN: C3 = 0.1 C2 = 0.28 C1 = 0.62 C0 = 0.0

Step 5: Calculation of Effective SHGC (SEF) The effective SHGC (SEF) is calculated using the following formula: SEF = (C3 x PF^3) + (C2 x PF^2) + (C1 x PF) + C0 The SEF value obtained in step 5 is the effective SHGC for the fenestration with a permanent external projection.

Exceptions to the above method:

The ECBC 2017 also specifies two exceptions to the above method. If the center of glass shading coefficient (SC) multiplied by 0.86 or the SHGC of the glass alone satisfies the SHGC requirements for the overall fenestration area, they can be used instead of the above method.

In conclusion, the SHGC is an important metric that affects a building’s energy consumption and cooling load. The determination of SHGC values should be done by an accredited independent laboratory and labeled or certified by the manufacturer. The use of shading coefficient or SHGC of the glass alone as an alternate to determine SHGC values for the overall fenestration product is allowed and can be used in some cases.