Windows and Overhangs for Hot-Dry and Mixed-Dry Climates
Summary:
This resource covers technical best practices for windows and overhangs in Hot-Dry and Mixed-Dry climates. It includes U-factors and other window rating systems, as well as overhang types.
Document:
Windows
Best Practice: Specify efficient windows to control solar energy gains and to help reduce heating and cooling loads. Some Building America experts recommend that, nationwide, windows be used with a U-factor of 0.35 or lower and a SHGC of 0.35 or less. Note that ENERGY STAR qualification can be met with windows at less stringent ratings.
Windows are a prominent feature of any wall. High-performance windows can be an easy way to achieve ENERGY STAR qualification. Efficient windows will add expense to your project, but will provide tremendous value in comfort, durability, and energy savings. High-performance windows add so much to energy efficiency that smaller cooling and heating equipment can often be specified, which may recapture much of the cost. A voluntary rating system developed by the National Fenestration Rating Council (NFRC) provides performance information for about half the windows sold. The NFRC label contains ratings for the following features. You can find more information about the NFRC on the web at: www.nfrc.org.
· U-factors take into account the entire window assembly and rate how well the window prevents heat from passing through the window. The lower the U-factor the better the window performs at stopping heat flow. U-factors are the inverse of R-values used to measure the effectiveness of insulation. U-factor values for windows generally fall between 0.20 and 1.2.
· SHGC is the solar heat gain coefficient, which measures how well the window blocks heat caused by sunlight. The lower the SHGC rating the less solar heat the window transmits. This rating is expressed as a fraction between 0 and 1.
· Visible transmittance (VT) measures how much light comes through a window. VT is expressed as a number between 0 and 1. The bigger the number the more clear the glass.
· Air leakage through a window assembly is included on most manufacturers’ labels, but is not required. The AL rating is expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq.ft.) The lower the AL, the less the window leaks. A typical rating is 0.2.
· Another optional rating is Condensation Resistance (CR), which measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 1 and 100, with a higher value representing more resistance to the formation of condensation.
ENERGY STAR qualifies specific windows in addition to qualifying houses. For the southern climate region all windows, doors, and skylights qualifying for the ENERGY STAR label must have a U-factor rating of 0.75 or below and a SHGC rating of 0.40 or below. You need not use ENERGY STAR-labeled windows to qualify a total house for an ENERGY STAR label.
The Efficient Windows Collaborative operates a web site that can help designers and consumers choose windows. The web site includes a tool that allows users to analyze energy costs and savings for windows with different ratings. Visit the web site at: www.efficientwindows.org/index.cfm.
The web site also has fact sheets with comparisons for each state. These fact sheets could make effective marketing tools. Also described on the Web site is a book entitled Residential Windows: A Guide to New Technologies and Energy Performance (Carmody et al. 2000), which offers homeowners, architects, designers, and builders a fascinating look at the state of the art in window technology. Emphasizing energy performance, the book covers every aspect of window design and technology: the basic mechanisms of heat transfer; new products and rating systems; the effects of window frame material and installation; and how to make the best decisions when purchasing windows.
Overhangs
Best Practice: Design roofs with overhangs to shade and protect windows and doors. Overhangs may take the form of eaves, porches, or other design features such as awnings, pergolas, or trellises.
Single glazing is not recommended, but when a house has clear single glazing, light-colored interior shades, overhangs, and combinations of shading devices significantly reduce energy costs.
Reliance on any form of shading is not nearly as important when windows with a low solar-heat-gain coefficient are used. Using a low-solar-gain low-E coating results in great energy cost reductions for air conditioning, even with no shading. This is because the glazing itself provides the necessary control of solar radiation, so shading measures become less important in terms of energy use. However, in homes designed for passive solar heating, overhangs help to control seasonal heat gain and a low SHGC can be detrimental. For a description of the interactions between window performance and shading, see the Efficient Windows Collaborative Web site at www.efficientwindows.org.
Overhangs should be sized to account for differences in the angles of the sun during winter and summer. Precise overhang dimensions should be calculated for each latitude. Free and low-cost computer programs and tools are available to help. For example a free program telling you the angle of the sun for any point in the country is available at www.susdesign.com/sungangle/. Latitude, longitude, and elevation data can be obtained at www.wunderground.com. Overhang dimensions can be calculated at www.susdesign.com/overhang/index.html. For a listing of free and available-for-purchase energy models, including solar design tools, see DOE’s Building Technology Program Web site at www.eere.energy.gov/buildng/index.cfm?flash=yes. Click on Software Tools on the lower right side. A low-cost sun angle calculator is available from the Society of Building Science Educators at www.sbse.org/resources/sac/index.htm.
Overhangs also provide protection from rain, hail, and the effects of overheating and ultraviolet radiation on siding and windows.
Hot-Dry and Mixed-Dry Climates
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