Windows and Walls
A Double Standard in Energy Efficiency
By Bruce Lang
The big secret regarding energy efficiency in recreational facilities 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 property 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 to be the norm. (R refers to resistance to heat flow; the higher the number the better the insulation performance.) On the other hand, windows with low emissivity (Low-e) coated glass touting the coveted Energy Star designation, and whose insulation performance tops out at R-4, are celebrated by architects, contractors and building managers. These knowledgeable observers rightfully see such de facto energy-conserving windows as a substantial improvement over conventional insulating glass whose insulation performance cannot exceed R-2.
But why do we expect our buildings to contain R-19 insulated walls and at the same time are willing to accept R-4 windows? Such an energy conservation double standard exists because it is easier to be a wall than a window. Walls only have to insulate well.
Windows (specifically window glass) must be transparent and colorless, facilitating the transmission of natural daylight, while reflecting unwanted solar energy, decrease ultraviolet radiation that causes fading of building components and furnishings, reduce sound transmission and, of course, to whatever extent possible, insulate against heat loss. In addition, windows must open to provide ventilation and egress in emergency situations. Compared to walls, a window must simultaneously perform numerous functions, many of which are highly sophisticated.
If the year were 1960 instead of 2011, 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 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 cause unhappy and uncomfortable users and staff, all too often cold in winter and hot in summer while the facility is paying more than it 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. While such a drastic move might save some energy, it would negate the increasingly recognized benefits of daylighting, the ability to transmit natural light into buildings through existing openings in walls and ceilings. The benefits of day lighting include:
- Reduced use of artificial illumination.
- Reduced sickness and attrition on the part of regular users and staff.
- Increased wintertime passive solar heating on south-facing glass.
Since most existing window openings in typical facilities can, with only minor modifications, take advantage of daylighting, there is a big incentive to make those window openings perform better, rather than 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 that can cause 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 Energy Star window performance standards will require windows possessing the coveted 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 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 cost accordingly.
- Heat-reflective insulating glass containing a suspended 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 windows, fixed glass and glass door applications has heralded the end of an energy-efficiency double standard for walls and windows. Suspended film insulating glass is saving energy at the Fitness Center of Durham College in Whitby, Ontario, Canada; the Montgomery County Regional Swim Center, Rockville, Md.; and the Westminster Recreation Center in Westminster, Colo.
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