Boasting a savings of 12% whole house energy consumption savings it is tempting to immediately order new highly insulated windows for the whole house. But before you do, consider the payback. Sure, you will be snug as a bug inside the house but according to the Department of Energys Pacific Northwest National Laboratory (PNNL), it takes two decades or more for these highly insulated windows to provide a utility bill return on investment.
Based upon a study conducted at PNNLs lab homes in Richland, Washington, aluminum-frame double-pane windows, common in homes across the country, were replaced with newer, triple-pane windows, known as highly insulating windows. The triple pane windows lowered energy use in the test home by 12.2 percent. But the cost of the windows is estimated to take between 23 and 55 years to make up.
A savings of 12 percent on whole-house energy consumption is substantial, especially when youre talking about changing a relatively small percentage of a homes envelope, said senior staff engineer Graham Parker, a founder of the project. But the windows are expensive.
The team points out other factors besides cost though.
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. Its hard to put a dollar value on comfort and health.
Benefits were found in the summer too. The low-emissivity (low-e) coating reduced internal solar heat gain. This cut energy consumption by nearly 25% during peak air conditioning periods thereby creating less demand on the electrical grid.
The control homes double-pane windows had a U-factor of 0.68, the equivalent of a 1.47 R-value. The triple-pane windows block approximately 80 percent of solar heat gain in summer, compared to 30 percent for the double-pane windows. (The triple-pane windows had a solar heat gain coefficient of 0.19 compared to the double-pane windows 0.70.)
The team ran experiments twice in 2012, 10 weeks between February and April, and 6 weeks between July and August. There were no window coverings and thermostats were kept at 75 degrees in winter and 70 degrees in summer.