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| Low-E Window Selection |
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 Whether you're a homeowner or designer selecting Low-E windows, it is important to understand how window performance is measured in terms of Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), U-factor, and Air Loss (AL). Heat generally passes through a window through radiation, conduction, or air leakage. In Houston, you generally want windows that block heat rays while letting in as much visible light as possible. You also want your windows to act as good, air-tight insulators. Nearly half of the sun's rays are infrared, or heat rays, that you cannot see. Low-E windows have a thin metal coating that reflects these heat rays, as well as some of the visible light. The amount of heat radiation passing through a window is called the Solar Heat Gain Coefficient (SHGC). Around Houston, you want a low SHGC. The current energy code calls for windows to have a SHGC less than 0.40. The other nearly half of the sun's rays are visible light. The amount of visible light passing through a window is measured in Visible Transmittance, or VT. In most cases, the more natural light there is in a room, the more healthy the space will feel and the less artificial light you will need in the room. You generally want the highest VT you can find while meeting the SHGC requirement. There is no code requirement for VT, but VT for low-E windows generally ranges from 0.40 to 0.70. The remaining small amount of the sun's rays are ultraviolet. Ultraviolet are the rays that are harmful to your skin and cause your furniture and carpets to fade. New "solar control" or "spectrally selective" low-E windows are available, meaning they have a coating that is calibrated to block infrared and ultraviolet light while letting in as much visible light as possible. Besides radiation, heat also can enter through your windows through conduction, meaning the heat passing through materials by contact. U-factor is a measure of the insulating performance of the window. It is the inverse of R-value, the term by which the insulating value of your wall or ceiling insulation is measured. Double and triple pane low-E windows come with inert gases such as Argon/Krypton between panes, and with thermal breaks built into the frames. Both are low-conductive materials that prevent heat from being conducted between the inner and the outer parts of the window. The energy code calls for a window U-factor of 0.75 or less, although you can find Low-E windows with U-factors as low as 0.30. Air leakage (AL) is measured in cubic feet per minute of air leakage per square foot of window area. This takes into account the entire window glass and frame. The energy code calls for AL of 0.30 or less. A very useful and free software tool for selecting windows is RESFEN by the Lawrence Berkeley National Laboratory. For more information on window selection in Houston, see these Texas fact sheets on efficient windows. |
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Contact Overland today to discuss your upcoming construction and property projects. We are a professional firm that is dedicated to providing undivided attention and unsurpassed service to our clients. You can call us at the contact number below, or use the Quick Contact email form and we will reply within one day.
Universal Phone: 832-250-2899
Fax: 480-275-3051
Email: athiess@overlandassessments.com
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Three Texas Locations
League City, TX (Galveston)
2951 Marina Bay Dr. #130-356
League City, TX 77573
Pasadena, TX (Houston)
3910 Fairmont Parkway #272
Pasadena, TX 77504
Beaumont, TX
2204 Oak North
Nederland, TX 77627