When is a Window Upgrade Worth the Extra Cost?

There's no doubt that windows are getting better — much better. The double-glazed, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. units that looked so ground-breaking a few years ago seem merely ordinary now that Canadian, European and some U.S. manufacturers are producing high quality triple-glazed units.

Triple-glazed windows offer substantial benefits, including improved indoor comfort and lower energy bills.

But they aren't cheap. Homeowners who are already facing painfully high building costs have to weigh the costs against the benefits. Are triple-glazed windows really worth the added cost?

Jon Wyman recently posed this question in a Green Building Advisor Q&A as he weighed the benefits of an upgrade for a double-wall house in western Massachusetts.

Wyman had already been planning to use triple-glazed windows in most of the house but was looking at another $4,800 up-charge to put triple-glazed units in the rest of the openings.

Marvin triple-glazed windows, with a U-value of 0.24 and a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.31, would cost $963, compared with $583 for Marvin's double-glazed units with a U-value of 0.33 and an identical SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1..

Sorry, no simple answers
Fuel costs will be lower and the house will be more comfortable with all triple-glazed windows, writes GBAGreenBuildingAdvisor.com senior editor Martin Holladay. That much is a given.

But the real question, he adds, is whether lower energy savings can be quantified? "You'll need to use energy modeling software to answer that question," Holladay says. "It would be necessary to model your home with the two options, and compare the energy use data."

Christopher Briley agrees that energy modeling is the only reliable way to compare double- and triple-glazed window performance. Even offering a gut reaction is tough, he adds, without knowing more about the design of the house and whether windows are primarily for heat gain or for admitting daylight.

"Then there's the matter of how you feel about the investment," Briley says. "Let's shoot from the hip and say an upgrade in windows to triple-pane saves you $240 a year (using today's fuel costs). Is it worth it to you? They are higher quality. They will feel different, even look a little different. I'm afraid there are too many variables to tell you for sure what you'd save or not save."

But the advantages of triple-glazed units are still clear
Even without running energy software to predict actual savings, Stephen Thwaites of Thermotech Fiberglass Fenestration points to several advantages of triple-glazed windows.

On the qualitative side, Thwaites would expect more comfort and greater resistance to condensation. Especially with larger window sizes, having the inner layer of glass warmer makes a "big, big difference in comfort," he says.

Condensation should be less of a concern in new construction where ventilation can be more easily controlled. Even so, Thwaites adds, "any triple beats any double, although obviously a warm edge triple will have the least condensation."

On the quantitative side, the relative savings offered by really good windows can be "substantial" as wall thickness increases, he says.

Energy modeling that factors in solar gain and window U-factors is the best way to caluclate savings for a specific house. However, in simulations for one double-wall house, Thermotech predicted savings of 35% with triple-glazed units over low-e double-glazed units.

"This is likely pretty close to an upper limit for any savings claims," he adds.

Experience counts, too
Andrew Henry offers first-hand experience in comparing Marvin double-glazed and Thermotech triple-glazed units. He had both in his own house, located north of Ottawa.

Henry says he saw "quite a bit" of condensation on the double-hung units, while "for all intents and purposes I don't get condensation on the Thermotech triple-glazed windows."

"As for comfort, I'd much rather sit near my triple glazed windows on a cold night than my double glazed windows," he adds. "You can feel the convective draft looping around when sitting next to the double glazed windows on a cold night."

Consider the type of window as well
The type of window — double-hung vs. casement, for example — also has a bearing on comfort and energy savings.

"If you're looking at triple glazing because of energy performance, you might want to reconsider your choice of double-hungs," writes James Morgan.

"Depending on local wind conditions, casements can offer significantly better air-sealing performance; also they can be substantially smaller than double-hungs and still meet code egress requirements."

Along the same lines, John Brooks would consider reducing the number of operable windows and doors. Fixed windows not only cost less, he says, but are also better than operable units in terms of U-factor, air tightness, and durability.

"I know that a window gives a three-in-one package (light, view, and ventilation)," Brooks says. "Sometimes dividing the functions works too."

Windows are a sound investment
Unlike kitchen counters, cabinets, and other easily replaced building components, windows are difficult and expensive to change later.

"In 20 years, what will you regret about your specification decisions? Homeowners NEVER regret putting in too much insulation or buying high-performance windows," Holladay says. "However, some homeowners get tired of the bathroom fixtures or kitchen cabinets. You might want to scrimp elsewhere, and invest in the good windows."