The Science of Windows

We all use windows every day but rarely appreciate the inner workings of a window. A window must

• Let light in.
• Keep the outside outside.
• Provide ventilation.
• Add to the aesthetic of a building.
• Avoid compromising the structure or energy efficiency of the wall it is in.

That is a lot to ask of any one material. Yet window technology today not only performs all these functions, but, in some cases, a window can be more energy efficient than the wall it is in.

Second only to the HVAC system in a house, windows are high-tech building components. The Greenbelt, Md.-based National Fenestration Rating Council developed a testing protocol to determine how well window components work. The result is a handy little sticker that graces new windows to explain their efficacy. Every new window has one (unless the manufacturer doesn’t get its windows tested). Unfortunately, very few building professionals (and homeowners) understand what the sticker means.

Energy Efficiency

Before we dissect the sticker, we must understand what makes a window energy efficient. There are three elements of a window that determine its energy efficiency: the frame material, glazing and spacers that keep the glass apart. Each element has multiple variables, and all three elements must work together to achieve high-performance characteristics.

Frame Material

Wood has been the material of choice for decades. It is fairly insulative, attractive and stable as a frame. It also requires maintenance because wood is susceptible to the elements and UV radiation. As a result, many companies clad the window exterior with aluminum or vinyl.

Vinyl is the most widely used window-frame material. It is less expensive than wood or aluminum, has similar insulative qualities to wood and can be extruded into any configuration. There is a wide variation in the quality of vinyl windows, however. You get what you pay for, and that can be a problem down the line for homeowners. Cheap vinyl windows contract in the cold and expand in hot weather. This can break the seal around the glass. Some vinyl windows today are filled with spray foam that adds insulation.

Fiberglass frames are the choice of many high-performance builders. They often are foam-filled and have a high R-value. They are very stable, and expand and contract at the same rate as the glass they support. They do not photo degrade and come in a multitude of colors that weather well.

Aluminum windows are extremely durable. Aluminum is a good conductor of heat so they have a very low R-value. When used in northern climates, aluminum windows should be thermally broken to reduce heat loss. They are much more prevalent in commercial construction. Aluminum also expands and contracts in thermal extremes.

Glazing

Glass goes back to the Roman era and didn’t change much for hundreds of years. The house I live in was built in 1972 in the Colorado mountains and had single-glazed (R-1) window walls. The house has fabulous views but was so cold that we couldn’t sit in the living room in the winter. When you add a second pane or sealed insulated glass, you can reach nearly R-2 (U-factor = 0.5).

For several decades an R-2 window was the best the industry could provide. With the advent of low-E coating, window glass improved to close to R-3 (U-factor = 0.35). This remained the standard for many years. The Canadians and northern Europeans improved window performance by adding a third layer of glass, creating R-4 to R-5 windows (U-factor = 0.25 to 0.2), and started to export windows to the U.S. Soon thereafter a California company named Southwall developed a product called Heat Mirror. This consisted of two panes of glass with a thin plastic film suspended between them; all internal layers were covered with low-E coating. This lightened the window weight considerably and had the same performance of R-4 to R-5.