Types of Windows

Windows are commonly described according to their number of panes of glass and whether any type of tint or coating is applied to the glass to alter its performance. High performance windows include heat-absorbing, reflective and spectrally selective glazings.

Glazing options – A wide variety of glazing options are available to satisfy different types of design conditions and performance requirements. Glazing can be single, double or triple pane. It can be clear, tinted, reflective or have spectrally selective coatings such as low-emissivity (low-e) coatings. The space between panes of glass can be filled with an inert gas (typically Argon) that doesn’t allow heat to pass readily. Common glazing options include:

• Clear glazing – The familiar standard without any type of tint or coating designed to alter the visibility or energy performance of the glass.                
• Tinted glazing –  Tinted glass is often specified as an inexpensive way to reduce solar heat gain through windows. Often referred to as "heat-absorbing" glass, tinted glazings block solar heat by actually absorbing it in the glass itself. However, while this does reduce the direct solar heat gain, it also causes the temperature of the glass to rise. The most common colors are bronze, gray and green and they all block solar heat gain in about the same proportion. Black tint is the worst choice for cooling load reduction since it absorbs much more visible energy than heat energy.                
• Spectrally selective glazing  – Spectrally selective glazings are designed specifically to admit a higher level of visible light while still controlling solar heat. They do so by responding differently to different wavelengths of solar energy allowing for much clearer glass with good solar control. Spectrally selective glazings are available with a range of performance characteristics. This provides a high level of design flexibility as different coatings and performance characteristics can be selected for each orientation providing optimal solar control while maintaining a uniform appearance. Popular "low-emissivity" or "low-e" glazings are a type of spectrally selective glazing that is typically selected to provide enhanced insulation value, good visible transmittance and good solar control. Some low-e windows, however, can allow more solar heat to penetrate than others so it is important to evaluate glazing options against all of a project’s design criteria.                
• Reflective glazing – Semitransparent metallic coatings are applied to clear or tinted glass to provide a high level of solar heat control. However, they reduce cooling loads at the expense of daylight transmittance. They can also cause glare problems in the surroundings and unintended heat gain in surrounding buildings.

Window performance ratings – Window systems are rated using several performance parameters that allow designers to select a glazing option that satisfies a project’s aesthetic, comfort, daylighting and energy efficiency criteria. Several important window performance ratings are discussed below, and the following table compares typical ratings for different types of windows.

• U-Value –  The rate of heat flow through a window assembly due to the temperature difference between the two sides of the window. The lower the U-value, the greater the insulating value of the window.                
• Shading Coefficient (SC) –  The ability of glazing to block the sun’s radiant heat. The SC is the ratio of solar heat gain of a window compared to single-pane 1/8" clear glass. The lower the SC, the lower the solar heat gain through the window.                
• Solar Heat Gain Coefficient (SHGC) –  The fraction of solar radiation passing through a window as heat compared to the amount of solar radiation striking the window. The SHGC is becoming a standard performance metric and is becoming more common in the performance ratings of manufacturers. It is similar to the shading coefficient in that the lower the SHGC, the lower the solar heat gain.                
• Visible Light Transmittance (VLT) --  The percentage of visible light that passes through the window assembly. A high VLT indicates a greater fraction of incident natural light passing through the window. Reducing cooling loads by specifying lower SC and SHGC ratings needs to be considered in conjunction with the visible light transmittance of the glazing choice to achieve a balance between cooling load reduction and the desired natural light environment.                
• Ultraviolet Transmittance – An indication of the percentage of ultraviolet radiation striking the glazing that passes through it. Many energy-efficient glazings also reduce UV transmission.                
• Sound Transmission – Sound transmission properties are expressed as the "outdoor-to-indoor-transmission class (OITC)." The higher the OITC, the better its sound insulation properties.

Selecting a glazing – About half of the sun’s energy is in a form that is visible to the human eye and the rest is in a form that contributes only heat to the interior of a building. When selecting a glazing option a good rule-of-thumb is to choose an option that maximizes daylighting and occupant comfort, minimizes energy use, and still meets the project’s design and aesthetic criteria. As the chart shows, glazing options are available that meet a wide range of performance criteria. Here are a few glazing selection considerations:

• Double-pane windows, while more expensive initially than single-pane, offer better insulating qualities and comfort in perimeter zones, greater design flexibility, improved sound attenuation, and reduced cooling loads.                
• For the Arizona desert climate, the ideal window choice is often a spectrally selective window that transmits as much of the sun’s visible energy as possible (high VLT) and rejects as much solar heat as possible (low SC or SHGC).                
• For maximum solar control and reduction of cooling load, it is particularly important to use glazing with low a SC or SHGC on the east, west, and to a lesser extent, the south sides of a building.                
• Don’t assume that dark glass is the best solar control. Many dark glazings block more light than heat and thus only minimally reduce cooling load over other choices.                
• The larger the window area, the greater the need for better insulating qualities, lower SC or SHGC, and lower VLT to reduce glare.

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