Glazing Performance Parameters

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Visible Transmittance

Visible Transmittance (VT) is the percentage of visible light (measured with beam radiation) that passes at a normal incidence angle through the glazing of a fenestration system.

“Right-sizing” the light admission properties of building apertures through the specification of glazing VT is a critical component of daylighting design. One major criterion for the selection of VT is whether the glazing unit is intended to admit daylight or provide view. (Insert Dual Function Window reference) Further considerations for fine-tuning VT include sky conditions, the luminance of the surround, interior target illumination and luminance levels and the design of the requirements daylight delivery system. (Insert Effective Aperture reference)

For a double-paned Insulated Glazing Unit (IGU), VT values range from as low as 12% for gray tinted inner and outer lites and a low transmittance coating, to as high as 80% for low-iron clear inner and outer lites and a spectrally selective low E-coating

High VT maximizes daylight admission, but can become a net energy loss when coupled with large glazing areas, or glazing that receives direct sun. These factors raise the potential for thermal discomfort and glare and increased mechanical loads due to conductive and radiative energy transfer. Too low VT glazing limits daylight potential to a point where even large areas of glazing fail to provide adequate daylight to the interior. This can increase internal loads from electrical lighting as well as and from heat absorption of the glass substrate.

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Lead Author(s): Matthew Tanteri

Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) is the fraction of incident solar radiation admitted through fenestration, both directly transmitted and absorbed, and subsequently released inward.

“Right-sizing” the heat admission properties of building apertures through the specification of glazing SHGC is a critical component of envelope design since it directly impacts heating and cooling loads and the requirements of the mechanical system.

SHGC is expressed as a number between 0 and 1. For a double-paned Insulated Glazing Unit (IGU), SHGC values range from as low as .12 for gray tinted inner and outer lites and a low transmittance coating, to as high as .82 for low-iron clear inner and outer lites and a high transmittance low-E coating. SHGC may be used to express the heat admission properties of the whole window, including the effects of the frame; or alternately it may represent the center-of-glass, which accounts for the effect of the glazing alone. The SHGC of the whole window, frame included, is generally lower than center-of-glass SHGC.

A high SHGC reduces heating load, which is important in a heating dominated climate. A low SHGC reduces cooling load, which is an important consideration in cooling dominated climate. Although the determination of SHGC is heavily dependent on climate zone, it is also influenced by several others factors that include building type (skin or internally loaded), usage and occupancy, fenestration orientation and area, and external shading.

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Lead Author(s): Matthew Tanteri

Light to Solar Gain Ratio

Light to Solar Gain (LSG) is the ratio of visible transmittance to solar heat gain coefficient expressed as a dimensionless fraction.

The formula for LSG is as follows:

LSG = VT / SHGC


LSG Ratio Components (source: Matthew Tanteri)

A glazing unit with an LSG of 1.0 transmits equal amounts of light and heat into the building envelope via radiation. The higher the LSG, the greater a glazing unit’s spectral selectivity. For cooling-dominated climates, this is an important parameter in the reduction of cooling load and the sizing of the HVAC system. LSG values greater than one indicate more light, less heat. LSG values less than one indicate more heat, less light. Glazing units with LSG values that are 1.25 or above are categorized by the Department of Energy's Federal Technology Alert publication of the Federal Energy Management Program (FEMP) as “high performance” because of their spectral selectivity.

Glazing has an inherent conflict in that a high VT is usually accompanied by a high SHGC, and a low VT is usually accompanied by a low SHGC. By adjusting the balance of light and heat, low-emissivity (low-E) coatings are used to customize the thermodynamic properties of glazing to optimize energy flows for solar heating, daylighting and cooling.

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Lead Author(s): Matthew Tanteri

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