Guest Column - April 2011
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Design Corner

Windows and Walls
A Double Standard in Energy Efficiency

By Bruce Lang


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.