WINDOW TECHNOLOGIES: Glass

Solar Heat Gain Simplified view of the components of solar heat gain. Heat gain includes the transmitted solar energy and the inward flowing component of absorbed radiation.

 

Visible Transmittance Center-of-glass solar heat gain values of double pane units.

Solar Heat Gain Coefficient (SHGC)

The second major energy-performance characteristic of windows is the ability to control solar heat gain through the glazing. 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). Some radiation is directly transmitted through the glazing to the building interior, and some may be absorbed in the glazing and indirectly admitted to the inside. Some radiation absorbed by the frame will also contribute to overall window solar heat gain factor. Other thermal (nonsolar) heat transfer effects are included in the U-factor of the window.

Window standards are now moving away from a previous standard referred to as Shading Coefficient (SC) to Solar Heat Gain Coefficient (SHGC), which is defined as that fraction of incident solar radiation that actually enters a building through the entire window assembly as heat gain. To perform an approximate conversion from SC to SHGC, multiply the SC value by 0.87.

The SHGC is also affected by shading from the frame as well as the ratio of glazing and frame. The SHGC is expressed as a dimensionless number from 0 to 1. A high coefficient signifies high heat gain, while a low coefficient means low heat gain.

Solar heat gain is influenced by the glazing type, the number of panes, and any glass coatings. Solar heat gain of glazing ranges from above 80% for uncoated water-white clear glass to less than 20% for highly reflective coatings on tinted glass. A typical double-pane IGU has a SHGC of around 0.70. This value decreases somewhat by adding a low-E coating and decreased substantially when adding a tint (see figure to the right). Since the area of a frame has a very low SHGC, the overall window SHGC is lower than the center-of-glass value.

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