Window technology has evolved over the years to the point where windows can be selected not only for their aesthetic qualities, but also for their performance abilities. For example, windows can be made from laminated glass that resists impact from flying debris in hurricanes, have special coatings that control the amount of heat gain and loss, or prevent water spots and dirt accumulation.
An NFRC label on the window will contain the information regarding the glazing features of a window - U-value, Solar Heat Gain Coefficient (SHGC), and Visible Light Transmittance (VT). Generally, the lower the U-value, the better the window performs at preventing heat loss (or gain in hot climates). U-value is equal to the inverse of R-value. SHGC is the fraction of sunlight which is admitted through a window and released as heat indoors. It is expressed as a number between 0 and 1-- the higher the number, the more solar heat the window transmits. VT is the portion (between 0 and 1) of the sun’s visible light that is transmitted through a window.
Typically, distributors and retailers will stock windows with the glass that is recommended for given climates, but custom sizes and brands that require special order will allow a greater choice of glass features. The Efficient Windows Collaborative and Energy Star provide websites to aid in specifying the most efficient window for a given climate.
Typical Center-of-Glass Window Properties
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VT |
SHGC |
U-value |
| Double-pane clear |
0.82 |
0.78 |
0.46 |
| Low-E |
0.78 |
0.58 |
0.25 |
| Solar Control Low-E |
0.72 |
0.4 or less |
0.27 |
Specifications per Cardinal Glass, www.cardinalcorp.com/data/tsb/ig/IG05_05-08.pdf
Impact-Resistant Windows
Building codes have long required laminated “safety” glass at hazardous locations like sliding glass and storm doors and windows adjacent to tubs. New on the building scene is glass laminated with composites that provide enough strength to allow windows to withstand high winds, projectiles, or even bullets. This impact-resistant glazing can reduce the risk of window failure and personal injury or property loss during tornadoes, hurricanes, and explosions. When struck, laminated glass may crack or shatter, but the glass fragments tend to adhere to a plastic layer and stay in place. In windows designed for high wind areas, a bonded interlayer is designed not to break under the stress of several projectile tests that model the force of airborne debris in high winds. The interior sheet of laminated glass is also adhered to a layer that will keep fragments in place.
Specialty Window Coatings
Low-E and solar control low-E (also called spectrally selective) coatings can be used to boost the energy efficiency of windows. Low-E double pane windows, most common in cold and moderate climates, are more energy efficient than clear windows because the low-E coating reduces heat loss through the window.
Solar control glass, also called Low E2, is a good glass for hot climates because, in addition to improving the insulating ability of windows, it also limits solar heat gain by blocking passage of infrared and some ultraviolet rays. Solar control glass allows a higher level of visible light to pass through a window with less solar heat gain reduction than tinted window coatings.
Hydrophilic Coatings
Lastly, a new glass coating is available that sheds water more quickly than standard glass and resists water spotting and dirt accumulation. A thin, invisible layer of silicon dioxide, applied to a window’s exterior, makes the surface smoother than conventional glass and hydrophilic. Hydrophilic molecules attract water molecules and they blend together, making water a very good solvent in many situations. Therefore, rainwater collects in sheets, slides off glass quickly, and actually cleans the surface more readily than conventional glass.
One manufacturer combines the features of solar control glass with a hydrophilic coated surface called LoE2 Plus.
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In most U.S. climates, low-E coatings reduce heating costs associated with energy loss through windows by lowering the window’s U-value.
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Compared to tinted windows, solar control windows allow more natural light to enter the home, thereby reducing the need for and energy used by artificial lighting.
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Impact-resistant windows can prevent damage and injury by preventing panes from shattering under impact.
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High performance glass is available in all types and styles of window. Most window manufacturers offer high performance lines that incorporate low-E glass and many also offer low E2, or solar control glass. Dealers in coastal and other high wind areas will be able to offer wind resistant glazing in windows.
Laminated “safety” glass adds about 50% to the cost of a typical 3050 window. Windows with laminated glass capable of sustaining hurricane force winds can cost three times that of a standard window.
Low-E coatings used to add about $1 per square foot of glazing, however, in most climates this has become the standard. Solar control coatings (low E2 ) may be incurring a similar surcharge, but have been moving toward the standard in some markets.
Low-E coatings save energy in most U.S. climates. In a simulation of a home located in Boston, low-E coating saved $103 per year.
Simulations from the Efficient Window Collaborative for a home with vinyl windows in Tucson, AZ, indicated that solar control glass reduces heating and cooling costs by $65 per year over double-pane clear glass..
Impact-resistant glazing or other window impact protection methods are required in the building codes of some areas prone to hurricanes and tornadoes. They are required in coastal Florida and Texas. Laminated glass should comply with local building codes, applicable ASTM standards, Consumer Product Safety Standards and SGCC/ANSI Z97.1 - 1984 (Rev. 1994).
The International Energy Conservation Code (IECC) includes state-by-state provisions for the energy efficiency of windows. Requirements are outlined on the Efficient Windows Collaborative Website at http://www.efficientwindows.org/code.cfm.
Not Applicable
Window installations do not vary with the glazing selected, so all comparable windows would be installed by the same methods. Glazings, glass thickness, frame composition and size will change the weight of the window. Windows should be installed per manufacturer’s specifications, and should include exacting fastening and flashing details.
Warranty varies by window manufacturer. Manufacturers commonly offer 10-year limited warranties although limited lifetime warranties are not unheard of.
Impact-resistant windows improve the safety of window glass in areas prone to high winds and flying debris.
Low-E glass improves the energy efficiency of windows and can improve interior comfort and reduce the occurrence of condensation on windows.
Solar control glass allows high levels of natural light to enter the home, reducing the need for supplementary artificial lighting during the daytime. Solar control glass also reduces interior surface reflectivity which can prevent occupants from seeing outside at night. In some cases, using solar control glass can reduce cooling loads so greatly that cooling system capacity can be reduced or that glass area can be added without increasing cooling loads. Solar control glass offers the greatest energy savings in areas where cooling costs are higher than heating costs.
Hydrophilic glass exterior coatings can reduce the frequency of window cleaning.
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