Windows have long been used in buildings for daylighting and ventilation. Many studies have even shown that health, comfort, and productivity are improved due to well-ventilated indoor environments and access to natural light. However, windows also represent a major source of unwanted heat loss, discomfort, and condensation problems. In 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion, one-fourth of all the energy used for space heating and cooling.
In recent years, windows have undergone a technological revolution. High-performance, energy-efficient window and glazing systems are now available that can dramatically cut energy consumption and pollution sources: they have lower heat loss, less air leakage, and warmer window surfaces that improve comfort and minimize condensation. These high-performance windows feature double or triple glazing, specialized transparent coatings, insulating gas sandwiched between panes, and improved frames. All of these features reduce heat transfer, thereby cutting the energy lost through windows.
Window systems are comprised of glass panes, structural frames, spacers, and sealants. In recent years, the variety of glass types, coatings, and frames available for use in window systems has increased dramatically, as has the opportunity to fine-tune and optimize window selection on a project-by-project basis.
Careful selection of window and glazing systems is essential to the energy efficiency and comfort of all buildings. Window and glazing choices should be considered holistically. Issues to consider include:
• Heat gains and losses
• Visual requirements (privacy, glare, view)
• Shading and sun control
• Thermal comfort
• Condensation control
• Ultraviolet control
• Acoustic control
• Color effects
• Energy requirements
Ultimately, the optimum choice of window and glazing systems will depend on many factors including the building use type, the local climate, utility rates, and building orientation.
Specifying Windows and Glazings
To fully specify a window system, it is necessary to specify the following characteristics:
• Window U-value
• Window Solar Heat Gain Coefficient (SHGC), or shading coefficient (SC)
• Glass Visible Transmittance
• Tints and Coatings
U-value indicates the rate of heat flow due to conduction, convection, and radiation through a window as a result of a temperature difference between the inside and outside. The higher the U-factor the more heat is transferred (lost) through the window in winter. The units of U-value are: Btus per hour per square foot per °F (Btu/hr · ft² · °F). U-factors usually range from a high of 1.3 (for a typical aluminum frame single glazed window) to a low of around 0.2 (for a multi-paned, high-performance window with low-emissivity coatings and insulated frames). A window with a U-factor of 0.6 will lose twice as much heat under the same conditions as one with a U-factor of 0.3.
Solar Heat Gain Coefficient (SHGC)
SHGC indicates how much of the sun’s energy striking the window is transmitted through the window as heat. As the SHGC increases, the solar gain potential through a given window increases. The SHGC is a ratio between 0 and 1. SHGC = 0 means none of the incident solar gain is transmitted through the window as heat and SHGC = 1 means all of the incident solar energy is transmitted through the window as heat. A window with a SHGC of 0.6 will admit twice as much solar heat gain as one with a SHGC of 0.3. Typically, windows with low SHGC values are desirable in buildings with high air-conditioning loads while windows with high SHGC values are desirable in buildings where passive solar heating is needed.
Tints and Coatings
The properties of a given glass can be altered by tinting or by applying various coatings or films to the glass. Glass tints are generally the result of colorants added to the glass during production. Some tints are also produced by adhering colored films to the glass following production. Tints are usually selected for aesthetic purposes. Some tints also help reduce solar gains. Coatings, usually in the form of metal oxides, can also be applied to glass during production. Some of these coatings, called “low-emissivity” or “low-e,” help reduce radiant heat transfer between panes of glass by blocking some or all of the IR wavelengths. These coatings can dramatically lower the window U-factor.
Other important attributes of window and glazing systems include:
• Gas Fills—Inert gases such as argon and krypton are often injected between panes of glass to reduce conductive and convective heat transfer. These low-cost, gas fills reduce U-values without affecting shading coefficients or visible transmittance.
• Fritting—Baked on ceramic coatings, or frits, can be applied to the surface of glass in many different patterns, colors, and densities.
Using high-performance windows can dramatically reduce heating and cooling loads, and eliminate the need for perimeter heating in internal-load dominated buildings due to the effect of increased Mean Radiant Temperature (MRT) on occupant comfort (see High-Performance HVAC). Window systems with low-e and spectrally selective coatings can filter damaging UV wavelengths and increase the life of room furnishings. Optimized fenestration systems for passive heating in residential buildings or for daylighting in commercial/industrial buildings will reduce loads and save operations and maintenance costs.