Windows, doors and other breaks in a home’s maximum energy envelope are always a trade off between energy efficiency and livability. Living in home without windows would be most depressing…but, even the most energy efficient window or skylight cannot compare as a thermal barrier to a well-insulated wall or ceiling. So, when you install a window in a wall, you are effectively cutting a hole in the home’s best thermal barrier. In other words, a well-built window might have an R-value of R-3, whereas exterior walls in a home commonly have an R-value of around R-20…that means the window is 17 points less efficient than the wall. This leaves the window to be a big heat loser when compared to the wall.
If you have been shopping for new or replacement windows, you may have encountered several confusing terms created to demonstrate the ability of a material or combination of materials to isolate one environment from another. In other words, "keeping the outside out and the inside in."
Many people are familiar with the term R-value, a term created to rate a particular material on its ability to resist the flow of “heat” through it. R-value is normally associated with wall or attic insulation material. When addressing the benefit of commonly used insulation materials such as fiberglass batts or blown in cellulose and fiberglass. It is common to say, “four inches of this or twelve inches of that gives an R-value of R-19 or R-38. The larger number indicates more resistance to the flow of heat from one area to another.
U-value is a term created to indicate the thermal efficiency of several materials put together as a unit, such as a window unit, skylight or door assembly. When comparing R-value to U-value, a higher R-value is deemed better, while a lower U-value is the preferred rating. An R-value of 30 is better than an R-value of 20 and a U-value of 0.50 is better than a U-value of 0.75.
Low-e coating enhances the ability of a window to reflect or limit the thermal radiation from the Sun from penetrating the window. This accomplished with a coating applied to the glass, much like tinted windows in a car. This term is straightforward and easy to understand. Low-e coating is most desirable for clear sunny climates. Low-e also limits fading and sun bleaching in fabrics and carpets.
I often consider these terms to be a smoke screen employed by window manufactures to confuse the average person when they are shopping for home windows. These terms imply that window, doors and skylights are very efficient units. This assumption is simply not true. Some are merely better than others by a small amount.
If you are building a new home or purchasing a used home in today’s energy market, it is wise to consider the thermal efficiency of a home. Windows, skylights and doors are a big factor in maintaining a comfortable environment within the living area. Large windows and big doors equal a larger energy bill.
My mother summed it up years ago with the phrase: “What…you own the power company or something…Turn off that light.”
Jim McDonald is a retired general contractor and active real estate home inspector. Read more about sealing up drafty windows at this website.
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