What is SHGC of glass and how it effects in building heat gain ?

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The solar heat gain coefficient (SHGC) is the fraction of incident solar radiation admitted through a window. It measures how well a fenestration blocks heat from the sun.The lower the value of  SHGC, better are the products in blocking unwanted heat gain. Solar heat gain through windows is a significant factor in determining the cooling load of many commercial buildings. The origin of solar heat gain is the direct and diffuse radiation coming from the sun and the sky (or reflected from the ground and other surfaces). SHGC is  expressed as a number between 0 and 1. For Example, if a glazing allows 30% of incident heat to pass through it , then the shgc of glass is 0.3

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SHGC accounts for incident solar radiation. It means , the heat gain read by SHGC value is due to all direct components of sun, which is easily controlled by provision of external shading devices or any other external obstructions. Some radiation absorbed by the frame will also contribute to overall window solar heat gain factor. Other thermal (non-solar) heat transfer effects are included in the U-factor of the window.Therefore, SHGC shall be determined for the entire fenestration product (including sash and frame), and not just for the glass.Therefore, besides glazing properties, the type of window and shading devices can affect the SHGC calculation of fenestration.

shgc-of-glass

Window standards are now moving away from a previous standard referred to as Shading Coefficient (SC) to Solar Heat Gain Coefficient (SHGC).To perform an approximate conversion from SC to SHGC, multiply the SC value by 0.87.

energy-efficiency-thermal-glass

Energy Efficiency Thermal Glass

  • Calculation of effective SHGC –

There are are two important components that need to be determined to calculate the SHGC of a window. These are projection factor (PF) and corresponding M-factor.

  1. Projection Factor

It is the ratio of the depth of the shading device and the height of window.PF is calculate to determine  how well an overhang  or vertical fin shades the building glazing.

Projection-Factor

Projection Factor

Where,

A = Depth of shading (the distance from the window to the farthest edge of overhang or vertical fin)

B = Height of window in case of overhang, and width of window in case of vertical fin

2. Multiplication Factor

Whenever the fenestration is provided with overhangs and/or vertical fins, a separate “M” factor shall be determined for each orientation and unique shading condition. The multiplication factor is derived from projection factor , and is calculated based on four ranges of PF ( from 0.25-0.49, 0.5-0.74, 0.75-0.99 and 1.00 or more)

The SHGC of a window having an external shading device can be calculated by multiplying the SHGC value of glass with “M” factor read from the table below for different projection factors for different orientations.

shgc-m-factor-table

Effective SHGC = Maximum SHGC / Multiplication factor (M)

The overall thermal performance of windows can be evaluated on basis of following two energy performance parameters of windows.

a). Maximum U Factor (thermal transmittance)

b).Maximum SHGC (heat gain through direct solar radiation)

It has also been observed that SHGC of glass has greater impact on heat gain as compared to U value of glass.

 

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