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.


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.

Today, Sunnyvale, Calif.-based Serious Energy owns Southwall’s technology and is producing SeriousWindows. Fixed glass can be as high as R-11 (U-factor = 0.09) and operable units are up to R-7 (U-factor = 0.14). This represents a revolution in window technology. I installed 20 of these windows in my house more than one year ago, eliminating the R-1 windows. Now the windows in my 2- by 4-walls have a higher R-value than the wall itself. Other manufacturers are getting into high-tech glass, as well. Owens Corning has a new line of Heat Mirror fiberglass windows on the market. Others will follow suit.


As the parts of the window increase in energy efficiency, the spacers that hold the glass apart become increasingly important. For decades, aluminum has been the material of choice because it is inert and won’t corrode in the air gap. Even Heat Mirror windows used aluminum because it can resist the tension of the plastic film and keep it taut. Today, most windows use a variety of materials that are more resistant to heat flow, so the entire window acts as an efficient system.

NFRC Window Ratings

Now we can start to understand what the NFRC window sticker means. There are three key elements to the sticker: U-factor, solar-heat-gain coefficient (SHGC) and visible light transmission.

The upper right corner identifies the U-factor of the entire window as a system. The U-factor is resistance to heat flow—the inverse of the R-value, which is insulating value. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. If you divide the R-value into 1, you get the U-factor. For example, an R-2 window has a U-value of 0.5. A low-E window has an R-value of 3 or a U-factor of 0.33.

The second box is the SHGC. That value is between 0 and 1 and identifies how much infrared light (or heat in the light) gets through the glass. If the value is 0.33, then only one-third of the heat in the light gets through the glass. This is very helpful if the windows face east or west. Most windows come standard with low solar-heat gain, especially those distributed in the South.

The third box is the visible light transmission. Again, the value is between 0 and 1. It is similar to the SGHC in that it is the percentage of visible light that gets through the glass. Think of it as sunglasses. The visible light transmission value only really matters if you have two different types of glass next to each other because you will see the difference between the two. If all the glass in the room has the same value you will never know if it is low or high visible transmittance.

Some window manufacturers will let you choose the values of the glass by orientation, such as low solar-heat gain on east and west and high solar-heat gain on properly shaded southern windows to take advantage of passive solar-heat gain in winter.

Window Installation

A homeowner can purchase the best high-performance windows, but it is wasted money if the windows are not installed properly. When I had my windows replaced, the contractor insisted that he had installed thousands of windows and his crew knew what it was doing. When the crew showed up with batts of fiberglass and no spray foam, I knew I had some educating to do.

When installing windows—new or retrofit—it is important to seal the gap between the rough opening and the window frame with low-expansion foam, not fiberglass. Be careful of foams that expand too much and can put pressure on the frame. This will keep the window from operating properly. Low-expansion foams (roughly three-times expansion) fill the gap without putting too much pressure on the frame. The entire gap should be filled from the inside of the frame to the outside. Excess foam can be trimmed off after it sets up.
Install flexible window flashing after the housewrap is installed. The first strip goes under the window sill and extends 6 inches beyond the sill on both sides. Then the vertical strips go on either side of the window extending 6 inches above the window.

The next step is where the errors typically occur: The housewrap needs to be cut at the top of the window and folded up. The head flashing goes over the top of the two side flashing strips covering them from edge to outside edge. Then the housewrap is folded down to cover the head flashing. This ensures there is mechanical flashing around the window, and you are not depending on adhesive to waterproof the installation. I’ve traveled across the country and seen all four strips of window flashing installed over the housewrap.

Happy Endings

The bottom line is high-performance windows are one of the greatest gifts you can give your clients. They don’t know what they don’t know, and what they don’t see is what is most important. A window is a window to them. In extreme weather—hot or cold—the comfort that good windows provide is priceless. It is only after the fact that homeowners will come to appreciate it. After all, comfort is the only reason we live in houses, so why not give them the best?