Achieving Superior Energy Performance, Part II

A Look at How Windows and Glazing Affect a Building’s Performance

By George Sullivan

In my last article, “Achieving Superior Energy Performance, Part I,” I discussed how area-to-volume ratio, framing, and walls and ceilings affect a building’s thermal efficiency. In this article, I’d like to put windows and glazing into an example building shell and compare how these openings can impact thermal efficiency.

Considerations
There are three major considerations when placing windows and glazing into a building shell.

Framing options for the windows and glazing to the exterior walls, which can create additional thermal bridging
Window unit U-values, which represent the overall R-value of the window unit or glazing system (1/U = R)
Percentage of windows or glazing system effects on the average wall R-value as expressed in window-to-floor area

I will focus on three window-to-floor area ratios (25 percent is the example building as designed; 18 percent, which is the ENERGY STAR limit; and 15 percent, which is the IECC 2009 limit) and the effect of three common U-values (0.28, 0.35 and 1.0) in each example. (Remember: The lower the U-value, the higher the R-value of the window or glazing system).

All the U-values in the examples are for the entire window or curtainwall system—not mid-glass, which is the most commonly quoted U-value by manufacturers. Designers and consumers should request the technical data sheets for the entire window or curtainwall system.

The money spent on windows as related to energy performance and thermal comfort can be seen in our examples as related to the effective average R-value of the wall system. Lower wall-system R-values increase energy consumption and decrease thermal comfort because of air movement in the room from warm wall surfaces to cold wall surfaces, which is known as a convection air loop. The percentage of glazing also has a major effect on the average R-value of the wall system; the higher the percentage of glazing, the lower the effective average R-value of the wall system, even at low U-values for the window or glazing system.

Framing Changes and Thermal Bridging
Let’s look at possible framing of an example room and how those framing options can create thermal-bridging issues. Let’s assume we have a room with 100 square feet of floor area and 10-foot ceilings. The room would measure 10 by 10 by 10 feet with one exterior wall and a window centered in that wall, creating a window-to-floor area ratio of 25 percent. The surface area of the exterior wall in our example room is 14,400 square inches, or 100 square feet.

In a conventionally framed 10- by 10-foot wall section, framed with wood or steel 2 by 4s, the windows feature a king and jack stud on each side with a double 2- by 8-header, a double 2- by 4-window-sill plate and an additional cripple on each side supporting the window-sill plate. The window in the center of the wall changes the framed wall surface area to 10,800 square inches, and the thermal-bridging area of the framed wall now is 2,275.13 square inches—an increase of 238.88 square inches, or 21.1 percent.

If we use advanced framing for the wall, framing it at 24 inches on center, the wall has a thermally bridged area of 1,597.5 square inches, or an 11.09 percent thermal bridge. Therefore, advanced framing has a 21.54 percent decrease in thermal bridging. (See the graphs at the bottom of “Achieving Superior Energy Performance, Part I.”)

Advanced framing for the windows requires a king stud on each side with a double 2- by 8-header and a double 2- by 4-window-sill plate, both of which are thermally broken. The header and window sill are nailed through the king stud and have a structural bracket for additional support. The advanced framing reduces thermal bridging by 576 square inches per window opening compared with standard framing of 16 inches on center in our example wall.

As you can see, the framing options drastically changed the wall performance with and without the window. Without the window and using traditional framing, the IECC average wall R-value is 19.96, and the ASHRAE average wall R-value is 10.65. The IECC average wall R-value is 53.3 percent better than the average ASHRAE R-value.

With traditional framing and the window at a U-value of 0.28, the IECC average wall R-value is 9 while the ASHRAE average wall R-value is 6.68. The IECC average wall R-value has a 25.7 percent better performance than the ASHRAE average wall R-value. See the tables below.

Therefore, in our example, a window with a U-value of 0.28 and a window-to-floor area of 25 percent has decreased the average wall R-value by the following:
IECC average wall R-value = 9.0, or a decrease of 54.9 percent
ASHRAE average wall R-value = 6.68, or a decrease of 37.27 percent

Applying the Example to a Model Building
Let’s review the effects of all three window-to-floor area ratios on a project. The model building wall surface area is 11,376 square feet. The total conditioned floor area is 12,375 square feet. (See the graphs below for calculations.)

Our example room has a 25 percent window-to-floor area of 3,093.75 square feet.
U-value of 0.28/R-value of 3.57
IECC               9.00 average wall R-value
ASHRAE        6.68 average wall R-value

U-value of 0.35/R-value of 2.85
IECC             7.00 average wall R-value
ASHRAE       5.98 average wall R-value

U Value of 1.00/R-value of 1.00
IECC               3.43 average wall R-value
ASHRAE        3.03 average wall R-value

An ENERGY STAR example has an 18 percent window to floor area of 2,227.5 square feet.
U Value of 0.28/R-value of 3.57
IECC                10.57 average wall R-value
ASHRAE           7.43 average wall R-value

U-value of 0.35/R-value of 2.85
IECC               9.32 average wall R-value
ASHRAE        6.79 average wall R-value

U-value of 1.00/R-value of 1.00
IECC              4.46 average wall R-value
ASHRAE       3.78 average wall R-value

Under IECC 2009 requirements, the window-to-floor area ratio is 15 percent, or 1,856.25 square feet.
U-value of 0.28/R-value of 3.57
IECC                11.42 average wall R-value
ASHRAE           9.93 average wall R-value

U-value of 0.35/R-value of 2.85
IECC               10.20 average wall R-value
ASHRAE          8.99 average wall R-value

U-value of 1.00/R-value of 1.00
IECC              5.11 average wall R-value
ASHRAE       4.79 average wall R-value

By reviewing the effects of window-to-floor area percentages with three standard U-values in the example building, the U-value of 0.28 provides the best performance across all three window-to-floor area percentages. The window-to-floor area that reduces the average wall R-value the least is 15 percent with a U-value of 0.28; it has an IECC average wall R-value of 11.42 compared to the average wall R-value of 19.96 before we inserted the windows into the building shell.

The example project with the 25 percent window-to-floor area has the worst-performing building shell. Even at the lowest U-value of 0.28, the IECC average wall R-value is 9.00 compared to the average R-wall value of 19.96, a 54.9 percent decrease in energy performance versus a window-to-floor area of 15 percent with a window U-value of 0.28. The example building will use 12.2 percent more energy based on the building-shell design.

George Sullivan is founder, chief executive officer and senior principal of Eco Smart Building PC, Chicago. He is a member of Eco-Logic’s advisory board and available to answer your questions in Sage Advice.

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Achieving Superior Energy Performance, Part II

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