Dallas Proposed Reflectivity Change
The city of Dallas wants to limit the reflectivity of glass used on commercial buildings in the wake of the Museum Tower controversy.
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SECTION 426 ATTENUATION OF SOLAR REFLECTIVITY IMPACT ON NEIGHBORING PROPERTIES 426.1 General. Reflective material used on the exterior of buildings can result in undesirable glare for pedestrians and potentially hazardous glare for motorists. Reflective materials can also impose additional heatload and discomfort glare on other buildings. 426.1.1 Scope. This section applies to the construction of new and retro-fitted buildings and restricts the reflection of sunlight from buildings to surrounding areas and existing buildings. Exceptions: 1.These provisions do not apply to One- and Two-Family dwelling properties as defined in the Dallas Development Code, any residential or commercial structure which does not exceed three stories or 49 feet in height, industrialized buildings, industrialized housing, and manufactured houses. The following shall apply: 1.Prescription option. All parts of the building enclosure of a structure must have a maximum exterior normal specular reflectivity of visible light of 15%. Curved walls or facades and walls or facades that are curved-in-effect are excluded from this option and must use the performance option. 2.Performance option. A Reflectivity Report that analyzes the potential solar glare and solar glare intensity from the proposed new development on pedestrians, motorists, adjacent areas, landscape, air traffic and existing buildings will be required where alternative methods are proposed to meet the intent of this section. A registered Texas engineer who has demonstrable knowledge of the subject matter must certify that the report, plans and specifications comply with the requirements of this section. 426.2 Definitions. The following terms are defined: Building Enclosure. Refers to the building façade(s), connections, supports, roofs, and any other exterior building element. Emissivity. The emissivity is the measure of a surface’s ability to absorb or reflect far-infrared radiation. The lower the emissivity rating, the better the insulating qualities of the window system. Glare Reduction (internal). Glare Reduction is the reduction of visible light by a window system as compared to a window without an applied film. Reflectivity. The level of luminance or normal specular reflectivity of visible light.
Shading Coefficient. The shading coefficient is the ratio of solar heat passing through the window as compared to the solar heat gain that occurs through 1/8” (3mm) clear glass. The lower the number, the better the solar shading qualities of the window system. Solar Glare IntensityThreshold. Solar reflectivity originating from new or retro-fitted buildings on pedestrians, motorists, adjacent areas, landscape, air traffic, and existing buildings must be less than or equal to nominal sunlight conditions at any given time and day. Solar reflectivity report must quantify this effect. Solar Heat Gain Coefficient. Solar heat gain coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The lower the coefficient number for a particular window system, the better it is able to reduce heat. The SHGC is the fraction of incident solar radiation admitted through a window, directly transmitted and absorbed, then subsequently released inward. SHGC is expressed as a number between 0 and 1. Solar Radiation Absorption. The solar radiation absorption is the percentage of solar energy that is absorbed through the glass. The lower the number, the less solar radiation absorbed. Solar Radiation Reflectance. The solar radiation reflectance is the percent of solar radiation this is directly reflected by the glass to the outside of the building. The lower the number, the less solar radiation reflected. Solar Radiation Transmittance. The solar radiation transmittance is the percentage of solar energy in the solar spectrum directly transmitted through the glass. The lower the number, the less solar radiation transmitted. The spectrum of solar radiation is from 280nm to 2500nm (nanometers). Visible Light Transmittance (VLT). The visible light transmittance (VLT) is the percentage of total visible light that is transmitted through the glass. The lower the number, the less visible light transmitted. The spectrum for visible light is from 380nm to 780nm. Visible Light Reflectance. The visible light reflectance is the percentage of total visible light that is reflected by the glass to the outside of a building. The lower the number, the less visible light reflected. Ultra Violet Light. Ultra violet light is the percent rejection of invisible, high energy wavelengths emitted by the sun which is the primary cause of fading and discoloration of furnishings and materials, as well as causing skin to burn. U-Value. The U-Value is a measure of how well heat flows through an object (thermal conductivity). It is the rate of heat gain or loss through glazing due to environmental differences between outdoor and indoor air. It is dependent upon the local climate or environment where the window is located. It affects the level of heat transfer and the rate. The lower the U-Value, the better the insulation value of the glass.
Shading Coefficient. The shading coefficient is the ratio of solar heat passing through the window as compared to the solar heat gain that occurs through 1/8” (3mm) clear glass. The lower the number, the better the solar shading qualities of the window system. Solar Glare IntensityThreshold. Solar reflectivity originating from new or retro-fitted buildings on pedestrians, motorists, adjacent areas, landscape, air traffic, and existing buildings must be less than or equal to nominal sunlight conditions at any given time and day. Solar reflectivity report must quantify this effect. Solar Heat Gain Coefficient. Solar heat gain coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The lower the coefficient number for a particular window system, the better it is able to reduce heat. The SHGC is the fraction of incident solar radiation admitted through a window, directly transmitted and absorbed, then subsequently released inward. SHGC is expressed as a number between 0 and 1. Solar Radiation Absorption. The solar radiation absorption is the percentage of solar energy that is absorbed through the glass. The lower the number, the less solar radiation absorbed. Solar Radiation Reflectance. The solar radiation reflectance is the percent of solar radiation this is directly reflected by the glass to the outside of the building. The lower the number, the less solar radiation reflected. Solar Radiation Transmittance. The solar radiation transmittance is the percentage of solar energy in the solar spectrum directly transmitted through the glass. The lower the number, the less solar radiation transmitted. The spectrum of solar radiation is from 280nm to 2500nm (nanometers). Visible Light Transmittance (VLT). The visible light transmittance (VLT) is the percentage of total visible light that is transmitted through the glass. The lower the number, the less visible light transmitted. The spectrum for visible light is from 380nm to 780nm. Visible Light Reflectance. The visible light reflectance is the percentage of total visible light that is reflected by the glass to the outside of a building. The lower the number, the less visible light reflected. Ultra Violet Light. Ultra violet light is the percent rejection of invisible, high energy wavelengths emitted by the sun which is the primary cause of fading and discoloration of furnishings and materials, as well as causing skin to burn. U-Value. The U-Value is a measure of how well heat flows through an object (thermal conductivity). It is the rate of heat gain or loss through glazing due to environmental differences between outdoor and indoor air. It is dependent upon the local climate or environment where the window is located. It affects the level of heat transfer and the rate. The lower the U-Value, the better the insulation value of the glass.
