Windows and Walls, A Double Standard in Energy Efficiency
The big secret regarding energy efficiency in commercial buildings is window glass, which 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 direct solar radiation, knowledgeable facility 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 means 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 and inert gas fill that meet the coveted Energy Star® designation, and whose insulation performance tops out at R-4, are celebrated by architects, contractors, and facility managers. These knowledgeable professionals rightfully see such defacto energy conserving windows as a substantial improvement over conventional insulating glass with insulation performance of R-2.
But why do we expect our buildings to contain R-19 insulated walls, while at the same time we 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. Window must do a lot more.
Windows (specifically window glass) must be transparent and colorless, transmit natural daylight, reflect unwanted solar energy, decrease ultraviolet radiation that causes fading of building components and furnishings, reduce sound transmission, and of course, insulate against heat loss. In addition, many windows must also 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 2010 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% to 35% 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% of the total carbon emissions in the U.S. annually. In addition, inefficient windows and glass cause unhappy and uncomfortable occupants, 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 solution might be for facility managers to board up existing windows. And 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 office and retail buildings through existing openings in walls and ceilings.
Since most existing window openings in typical office and retail space 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, here’s what facility 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 separated by a sealed air space) with a solar heat-reflective coating is appropriate for buildings concerned with staying cool in 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 also reflect ambient heat both inside and outside, combined with inert gas fill (such as argon or krypton) 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 that qualify for 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 insulating glass consisting of three panes of glass and two heat reflective coatings. The good news is that adding a third pane of coated glass improves insulating glass performance. The bad news is that triple pane glass is 50% heavier than dual pane insulating glass, requiring stronger window framing, more difficult installation and handling of large units, and increasing cost accordingly.
- Suspended film insulating glass containing one or more transparent, heat reflective films suspended inside the air space. Because such films are practically weightless, up to three films can be used to create up to four insulating cavities that dramatically increase insulation performance while reflecting unwanted solar heat. Such glass can achieve a center of glass insulation value of up to R-20, comparable to that of an insulated wall.
Clearly, the advent of new high performance glass technologies for standard window and curtain wall applications has heralded the end of an energy efficiency double standard for walls and windows.
* Southwall’s Heat Mirror suspended film insulating glass has been selected to be retrofitted into all 6,500 windows in New York’s Empire State Building current energy upgrade. The film is being installed in replacement windows for the building by Serious Materials of Sunnyvale, CA. To read FacilityBlog’s April 2009 story on the project, click here.
* Southwall does not make windows. Rather, Southwall Heat Mirror fabricators make Heat Mirror insulating glass that is used in fixed glass, curtainwall and operable window and door applications.