How Energy Efficient Are Energy-Efficient Windows?

How energy efficient are energy-efficient windows? In terms of straight insulating value, the average Energy Star window today is more energy efficient than it has ever been. Energy efficient windows can...

Energy efficient windows can produce a significant reduction in a home's energy use and costs. Information provided by the University of Minnesota's College of Architecture compares energy savings between different types of windows in both Boston, Massachusetts, and Phoenix, Arizona.


In Boston the figures show the savings for heating costs, and in Phoenix for cooling costs. Compared to a single pane window in an aluminum frame, a double pane window in a wood or vinyl frame provided a 24% savings, a double pane with low-E coating provided a 30% savings, and a triple pane window with a low-E coating in an insulated frame provided a 36% savings in annual heating costs. The low-E, or low emittance coatings, mentioned are actually very thin layers of metal that are applied to the glass of the window pane. This coating allows the sun to heat the window so that it becomes an aid in heating the home rather than a source of lost heat.




In Phoenix, the figures show that a single pane window in an aluminum frame can provide an 8% energy savings over a single pane window in an aluminum frame with no tint. A double pane window in a wood or vinyl frame provided a 16% savings and a double pane window in a wood or vinyl frame with a low solar-gain emittance coating provided a hefty 32% savings in annual cooling costs.
Energy efficient window expert Steven Poitz of Thermotech explains how his company achieves the most energy efficient windows possible.

"In terms of straight insulating value, which is the way that people normally think of energy efficiency, the average Energy Star window today, which would be double glazed with a low energy coating, would insulate on average to about an R3 rating. A normal window that was not energy efficient would rate around R2. For Thermotech, Energy Star's criteria are the starting point. Most of our windows insulate 50% better and allow twice the passive solar gain of typical Energy Star windows. We have windows that use both the low energy coating and triple glazing and they insulate to about an R5 rating. However, insulating value is only part of the story. This is especially true if you live in a colder climate where your heating bill would be larger than your cooling bill because windows are a unique element of what we call the building envelope. Unlike a wall, a ceiling, or a floor, windows do not just lose energy. They can actually help heat your house especially if they face south. So the more of these energy efficient windows you have the lower your heating bill is especially if you are in a colder climate. These windows insulate quite poorly compared to your wall because they have the characteristic of letting in light and, in the end, heat. So we have this unusual situation that is counter-intuitive. You would think that the house with more windows would have a higher heating bill, but that is not necessarily true."

In order to understand Poitz's technical explanation you need to know that Energy Star ratings signify the energy efficiency ratio and that the higher the ratio the more energy efficient the product is. You should also know that glazing means a pane of glass. In other words a double glazed, or glazing, window has two panes of glass and a triple glazed window has three panes of glass.

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