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All glass, or glazing, may look the same. But different types don't perform the same. If the climate in your area includes blazing summer sun or frigid winter winds, you'll want a quality glass that offers more protection. That way, your house - and your utility bills - will be much more comfortable.
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Single vs. Double Pane
Single-pane windows don't provide much insulation against cold or hot temperatures. Double-pane (or double-glazed) windows have an airspace, typically one-half to one inch, sandwiched between two sheets of glass to provide an extra layer of insulation. If the space is too narrow, temperature is too easily conducted. If the space is too wide, convection currents are created that transfer heat or cold.
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Triple Pane
Triple-pane windows aren't as popular as they used to be, mainly because the extra layer of glass adds significantly to the weight and cost while only marginally improving the efficiency. Today's double-panes, especially those with advanced low-E coatings, provide a comparable level of energy efficiency. In addition, triple-panes are very heavy and require thicker framing to support their weight, which means less light and view.
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Air vs. Argon Gas Blend Insulation
In most double-pane windows, air trapped between the glass provides the insulation. In better double-panes, the space is filled with an inert argon gas blend. Argon has a thermal conductivity, or heat transfer rate, that is 30% lower than air, making it more efficient.
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Low-E Glass
Low-emissivity (low-E) coatings on window glass can alter the flow of heat and light transmitted through the window. Without this coating, clear-glass windows can cause heat build-up inside your home in summer. And in winter, the warmth inside a room can radiate to the outside.
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U-factors and R-values
These terms are often used by manufacturers in describing the thermal efficiency of a window. U-factor indicates how quickly heat is transferred through a window, and R-value is the opposite. So a window with a low U-factor, or a high R-value, is better at keeping homes warmer in winter and cooler in summer.
Confused? Here's an example: single-pane windows are usually given an R-value of 0.9, while double-panes may receive a 2. Renewal by Andersen® argon gas-filled, low-E double-panes consistently deliver R-values of 3 to 4, depending on the size and style.
To make things even more confusing, some manufacturers provide R-values for the center of the glass, but not for the entire window, including its frame. To help ease the pain, the National Fenestration Rating Council (NFRC) has created industry-wide standards to make comparisons accurate and easy. So if a manufacturer's glass has an NFRC label on it, you'll know what you're getting before you buy.
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