Window Glass: An Energy Loser

PALO ALTO, Calif. — One key challenge regarding energy efficiency in Laundromats is dealing with window glass, which, when compared to insulated walls and ceilings, is a terrible “energy loser.” While we expect that energy-conserving walls and ceilings will dramatically insulate against heat loss and block solar radiation, knowledgeable owners and distributors anticipate far less in the way of energy conservation from even the most energy-efficient windows.

FILLING NUMEROUS ROLES

The numbers speak for themselves. Walls with an insulation performance value of R-19 are considered to be the norm. R means resistance to heat flow. The higher the number, the better the insulation performance — keeping heat in during the winter and keeping heat out during the summer.

Windows with low-emissivity-coated (Low-e) glass touting the coveted ENERGY STAR designation, and whose insulation performance tops out at R-4, are celebrated by contractors and building managers. These observers rightfully see such energy-conserving windows as a substantial improvement over conventional insulating glass whose insulation performance cannot exceed R-2.

Why do we expect our buildings to contain R-19 insulated walls and at the same time accept R-4 windows? Such an energy-conservation double standard exists because it’s easier to be a wall than a window. Walls only have to insulate well.

Windows (specifically window glass) must be transparent and colorless (viewing the interior from the outside is a key security factor), facilitating the transmission of daylight, while reflecting unwanted solar energy, decreasing ultraviolet radiation that causes fading of furnishings, reducing sound transmission and insulating against heat loss in winter and heat gain in summer. Windows must also be able to provide ventilation and egress in emergencies. Compared to walls, a window must simultaneously perform numerous functions.

If it were 1960, perhaps we could maintain one energy-conservation standard for walls and ceilings and another less-demanding standard for windows and glass. But we can no longer afford to do so.

Despite insulated walls and ceilings, and the popularity of ENERGY STAR-designated windows, 25 to 35 percent of the energy used in homes and buildings is wasted due to inefficient glass. It should be no surprise that glass is responsible for less than10 percent of the total carbon emissions in the United States annually. Inefficient windows and glass contribute to cold stores in the winter and hot stores in the summer, resulting in unhappy customers. Owners also aren’t happy when inefficiency breeds higher heating-and-cooling costs.

TAKING ACTION

Glass is the heart of a window. When ordering windows, here’s what store owners need to know in terms of energy conservation:

• Single-pane glass may keep out the weather, but it does little to insulate against heat loss or reflect the sun’s heat, leading to overheating. In most locations, single-pane glass is not code-compliant.
• Insulating glass (two panes sealed together) with solar heat-reflective coating is appropriate for owners who are concerned with their store staying warm in the winter and cool in the summer. The air space inside the sealed glass enhances insulation, and the coating reflects the sun’s heat to prevent overheating.
• Insulating glass with dual heat-reflective coatings that simultaneously reflect heat from the sun and ambient heat both inside and outside is even more effective in saving energy and increasing occupant comfort.
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The window story doesn’t end here. Keep in mind that recent and impending revisions to the Department of Energy’s ENERGY STAR window-performance standards will require windows possessing the ENERGY STAR designation to provide increased energy efficiency.

Glass available today that will meet the new and forthcoming ENERGY STAR window-performance standards include:

• Triple-pane glass consisting of three panes and two heat-reflective coatings. The good news is that by using a third pane of coated glass, you improve insulating-glass performance. The bad news is that triple-pane glass is 50-percent heavier than insulating glass, requiring stronger window framing and increasing your costs accordingly.
• Heat-reflective insulating glass containing a transparent, heat-reflective film inside the air space can dramatically increase insulation performance while reflecting unwanted solar heat. Single-, double- and even triple-internal films that create as many as four separate cavities inside the insulating-glass unit can achieve a center of glass- insulation value of R-20, superior to an insulated wall.

The advent of new, high-performance glass technologies for standard window and fixed-glass applications heralds the end of an energy-efficiency double standard for walls and windows.