Glass Thermal Performance
In exterior wall specifications, several statistics of glass performance show up consistently. Clear descriptions of these properties can be found in the GANA Glazing Manual. Here are summaries of these performance properties.
Visible Light Transmittance is the percentage of visible light in the solar spectrum transmitted through a glass. It’s expressed as a decimal; i.e. 0.90 for clear glass or 0.40 for tinted/coated glass.
Visible Light Reflectance is the percentage of visible light within the solar spectrum reflected by the glass.
Solar Energy Transmittance is the percentage of ultraviolet energy within the solar spectrum transmitted through the glass.
Solar Energy Reflectance is the percentage of ultraviolet energy within the solar spectrum reflected by the glass.
Shading Coefficient is the ratio of the solar heat gain of a particular glass to the solar heat gain through a lite of 1/8” clear glass. 1/8” clear glass is given a value of 1.0
Calculation: S.C. = (Solar Heat Gain of Glass in question) ÷ (Solar Heat Gain of 1/8” Clear Glass)
Solar Heat gain includes the heat directly transmitted through the glass and the solar radiation that is absorbed and reradiated within the interior space.
U-value is a measure of air-to-air heat transmission (loss or gain) due to thermal conductance and the difference in indoor and outdoor temperatures. A lower U-Value means less heat is transferred through the product in question. Glass manufacturers publish center of glass U-values as the edges may have a higher U-value due to the insulating unit spacer. Window manufacturers publish total window U-values.
Expressed as: Btu/hr/ft/°F
R-value is the reciprocal of the U-value. This value measures the thermal resistance of a glazing system. The higher the R-value, the less heat is transmitted through the glazing.
Expressed as: ft²/hr/°F/Btu
Calculation: R-value= 1 ÷ U-value
Relative Heat Gain (RHG) is the amount of heat gain through a material, taking into account the shading coefficient and conductive heat gain.
Expressed as: Btu/hr/ft²
Calculation: RHG = (Summer U-value x 14°F) + (Shading Coefficient x 200)
Solar Heat Gain Coefficient (SHGC) is the ratio of the solar energy entering the space to the incident solar radiation. Manufacturers provide center of glass
Calculation: SHGC = (Solar Energy Gain through Glazing) ÷ (Solar Energy Incident on the Glazing)
Emissivity (e) is the measure of a surface’s ability to emit long wave infrared radiation.
Emmittance is the ratio of the rate of radiant emission. Is a consequence of temperature only.
Coolness Index is the (visible transmittance of a glazing system) ÷ (solar heat gain coefficient). This ratio is helpful for selecting glazing products for different climates. i.e. selecting glass that transmits more heat than light vs. those that transmit more light than heat.
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Reader Comments (1)
Nice blog and very useful set of definitions.