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.