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Technical Resources Thermal Performance Terminology Increasingly, thermal performance is an important criteria in building design. Glass manufacturers and fabricators have responded to this with an array of products that feature a range of thermal performance in a variety of ways. To understand the high-performance capabilities of today's architectural glass products requires a strong working knowledge of optical and thermal performance terminology. The following terms are commonly used to describe and analyze the performance of architectural glass products. Visible Light Transmittance. The percentage of visible light within the solar spectrum (390 to 770 nanometers) that is transmitted through glass. Visible Light Reflectance. The percentage of visible light within the solar spectrum that is reflected from the glass surface. Solar Energy Transmittance. The percentage of ultraviolet, visible, and near infrared energy within the solar spectrum (300 to 2,100 nanometers) that is transmitted through the glass. Solar Energy Reflectance. In the solar spectrum, the percentage of solar energy that is reflected from the glass surface(s). Shading Coefficient. The ratio of the solar heat gain through a specific fenestration to the solar heat gain through a lite of 1/8 inch (3 mm) clear glass. Glass of 1/8 inch thickness is given a value of 1.0; therefore, the shading coefficient of a glass product is the solar heat gain of the glass in question divided by the solar heat gain of 1/8 inch clear glass 1.0. U-Value. A measure of air-to-air heat transmission (loss or gain) due to thermal conductance and the difference in indoor and outdoor temperatures. As the U-value decreases, so does the amount of heat that is transmitted through the glazing material. A lower U-value reduces the amount of heat transferred through the fenestration product. Relative Heat Gain (RHG). The amount of heat gain through a glazing material taking into consideration the effects of solar heat gain (shading coefficient) and conductive heat gain (U-value). R-Value. The R-value is the reciprocal of the U-value. The higher the R-value, the less heat is transmitted throughout the glazing material. Solar Heat Gain Coefficient (SHGC). The ratio of the solar heat gain entering the space area through the fenestration product to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation which is then reradiated, conducted, or convected into the space. Emissivity. The measure of a surface's ability to emit long-wave infrared radiation or room temperature radiant heat energy. Coolness Index (Luminous Efficacy or Light-to-Solar Gain Ratio). The visible transmittance of a glazing system divided by the solar heat gain coefficient. This ratio is helpful in selecting glazing products for different climates in terms of those that transmit more heat than light and those that transmit more light than heat. This material has been adapted from the Glass Association of North America's GANA Glazing Manual. Source: Appeared in Glass Guide for Architects, Glass Magazine June 2000. |
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