Highly insulating windows are very energy efficient, though expensive

Highly insulating triple-pane windows keep a house snug and cozy, but it takes two decades or more for the windows to pay off financially based on utility-bill savings, according to a report by energy efficiency experts at the Department of Energy's Pacific Northwest National Laboratory.

The findings are being presented Dec. 4, 2013 by PNNL research engineer Sarah Widder at the Buildings XII Conference in Clearwater Beach, Fla. The meeting is sponsored by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers).

The report by Widder and senior staff engineer Graham Parker is based on a study at PNNL's Lab Homes, a pair of identical manufactured homes used to study energy efficiency. Researchers replace one item at a time in one home, such as the windows or the water heater, then compare the performance of the homes.

For this study, the PNNL team studied the effect of replacing aluminum-frame double-pane windows, which are common in homes across the country, with newer, triple-pane windows, also known as highly insulating windows. The team found that the newer windows slash energy use in the home by 12.2 percent. But because of the cost of the highly insulating windows, it would take anywhere from 23 to 55 years for the reduced energy cost to make up for the increased expense.

"A savings of 12 percent on whole-house energy consumption is substantial, especially when you're talking about changing a relatively small percentage of a home's envelope," said Parker, a founder of the Lab Homes project. "But the windows are expensive."

The team notes that there are many other factors to consider in addition to money when deciding whether to install highly insulating windows.

"Comfort is also important," said Parker. "The windows cut down dramatically on cold air radiating from the windows and they reduce temperature variations in the home, where some areas will be much warmer or cooler than others. They also nearly eliminate the formation of condensation on the inside of the window which can lead to mold growth and unhealthy indoor air. It's hard to put a dollar value on comfort and health."

The PNNL team found an added value during summer months: The newer windows, which have low-emissivity (low-e) coatings that reduce internal solar heat gain, slashed energy consumption by almost 25 percent during peak cooling times on hot summer afternoons. That benefit is passed along to utilities as less demand on the overall electricity grid at times when demand and price are highest, Widder said.

Lab homes - an energy laboratory in a home environment Each of the lab homes is about 1,500 square feet and is equipped with more than three dozen sensors which hang from the ceiling or are attached to windows, walls, and other areas throughout the home. The sensors record information on temperature, humidity, and the amount of sunlight entering through windows. Instruments outside the homes record temperature, humidity, wind speed, rainfall, barometric pressure, and wind direction.

The house has 196 square feet of windows, including two large sliding glass patio doors and eight other windows.

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Highly insulating windows are very energy efficient, though expensive