The big secret regarding energy efficiency in commercial buildings is window glass, which compared with 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 direct solar radiation, knowledgeable building managers and project developers anticipate far less in the way of energy conservation from even the most energy efficient windows.
The numbers speak for themselves. Walls with an insulation performance value of R-19 are considered the norm. (R means resistance to heat flow. The higher the number, the better the insulation performance; in winter, keeping heat in; in summer, keeping it out.) On the other hand, windows with low-emissive (Low-e) coated glass touting the coveted Energy Star designation, whose insulation performance tops out at R-4, are celebrated by architects, contractors and building managers. These experts rightfully see such energy-conserving windows as a substantial improvement over conventional insulating glass whose insulation performance cannot exceed R-2.
Great Expectations
But why do we expect our buildings to contain R-19 insulated walls, but at the same time are willing to accept R-4 windows?
Despite heavily insulated walls and ceilings and the popularity of Energy Star-designated windows, 25 percent to 35 percent of the energy used in homes and buildings is wasted due to inefficient glass. So, it should come as no surprise that glass is responsible for more than 10 percent of the total carbon emissions in the United States annually, and is a major contributor to global warming.
In addition, inefficient windows and glass create unhappy and uncomfortable commercial tenants, who are often too cold in winter, too hot in summer, and paying more than they should in heating and cooling costs year-round.
One might think the easiest solution for building managers would be to board up many existing windows. But such a drastic move would negate the increasingly recognized benefits of day lighting –– the ability to transmit natural light into office and retail buildings through existing openings in walls and ceilings.
Most existing window openings in typical office and retail space can, with only minor modifications, take advantage of day lighting. That means there is a big incentive to make those window openings perform better, not reduce their size and number.
Since glass is the heart of a window, when ordering new windows, here’s what property managers need to know about glass options:
• Single-pane glass may keep out the weather, but it does little to insulate against heat loss or reflect the sun’s heat to prevent overheating. In most locations, single-pane glass is not code-compliant.
• Insulating glass (two panes sealed together) with a solar heat-reflective coating is appropriate for buildings concerned with staying warm in winter, cool in summer. The air space inside the sealed glass enhances insulation, and the coating reflects the sun’s heat to prevent over-heating.
• 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.
Many might think this is where the story ends. However, recent and impending revisions to the Department of Energy’s window performance standards will require windows possessing the coveted Energy Star designation to increase energy efficiency.
Glass available today that will meet the new and forthcoming window performance standards includes:
• Triple-pane glass consisting of three panes of glass and two heat-reflective coatings. The good news is that by using a third pane of coated glass, triple pane improves insulating glass performance. The bad news is that triple-pane glass is 50 percent heavier than insulating glass, requiring stronger window framing and increasing 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.
Clearly, the advent of new, high-performance glass technologies for standard window and curtain wall applications heralds the end of an energy efficiency double standard for walls and windows.
Bruce Lang, blang@southwall.com, is vice president of marketing and business development at Southwall Technologies, in Palo Alto, Calif. Southwall Technologies develops and manufactures high-performance, energy-saving films and glass prodcuts.
