Understanding Energy Efficient Windows


Camilla McLaughlin — For The Associated Press

More panes, more gains. In fact, the newest windows can be almost as energy efficient as a solid wall, and even repel dirt and water, making them almost self cleaning.

The Department of Energy is in the final stages of completing new requirements for Energy Star ratings for windows to establish higher levels of performance. The changes will be rolled out in two phases beginning this summer at the earliest. The second, more stringent set will take effect in 2013.

The new ratings divide the United States into six different climate zones establishing higher standards for each depending upon the demands of the region's weather.

"There are very few windows today that will meet those standards and more manufacturers will have to go to triple glazing or higher," says Denny Giantomasi, executive vice president of Empire Building Products in Schuylerville, N.Y.

Consumers can easily be overwhelmed, but having so many options means no matter the goal, space or budget there is a window that will fit the bill.

There are five standard types of windows:

— Awning windows are hinged at the top and open out like an awning. They are often placed above or below other windows or over doors for extra ventilation and light.

— Bay and bow windows reach out from the wall and are a good way to capture views even if wall space is limited.

— Casement windows have one hinged sash that swings out.

— Double hung windows have two sash that slide up and down; the bottom sash can be raised to bring in cool air while the upper sash can be lowered to pull hot air from the ceiling outside.

— A single hung window has the same timeless style as double hung but only the bottom sash is movable.

Beyond these basics, windows can be round, angled curved or shaped and can be custom made to any specifications. The design trend for larger windows, meanwhile, has put pressure on manufacturers to create windows that offer the same protection as walls without compromising views or light.

Windows have come a long way since the days of a single pane of glass in a wood frame. Efficiency is judged by several measures including solar gain, the degree to which a window transfers heat (U-factor), air leakage, and how much visible light is transmitted through the glass.

As new window designs do an even better job at insulating, some manufacturers are expressing their windows efficiency in R-values, which show how well a material resists heat transfer.

The R-value is basically the opposite of the U-factor. A single pane of glass, for example, has an R-value of 1.2 and a U-factor of 0.84, while a double pane has an R-value of 2 and a U-factor of 0.049.

In climates with hot summers and cold winters all of these factors may be equally important, while in others solar gain might be more important than a window's U-factor.

Phase one of the proposed new Energy Star ratings call for U-factor ratings calls for U-factors ranging from 0.30 to 0.50. For phase two, in the coldest regions, potential U-factors could range from 0.19 to 0.28, depending on the amount of solar gain.

While frame materials and installation play a role in a window's efficiency, the most important aspect is the glass and how it is assembled in the frame. Until recently, double glazing, two panes of glass, were the industry standard. The addition of a special low-emission coating to the glass further reduced heat loss while an inert gas such as argon or krypton was pumped between the panes to add even more insulation.

In the last year, most major manufacturers have added new energy coatings and also triple panes to their product lines.

In upstate New York, Empire Building Products saw sales of triple glazing increase by 30 percent last year, and like many in the industry, the company anticipates that by 2015 triple glazing could become an industry standard in some of the coldest parts of the country.

However, instead of adding a third pane — which adds weight — some manufacturers are using suspended films between two panes to increase efficiency.

Serious Materials, based in Sunnyvale, Calif., for example, has created a window that is four times more efficient than current Energy Star requirements, and the company says it is also one of the few windows that exceeds Energy Star standards proposed for 2013.

In addition to insulated glass and a noble gas fill, these windows have a plastic film suspended between ultra wide insulated glass, a fully insulated frame systems and a fiberglass frame with low conductivity. The windows and film are matched not only to the local climate but also to the location of the window on the house.

Serious Materials estimates an annual savings of 40 percent in energy costs for their top-end window, which has an R-value of 11.8 and a U-factor of 0.09.

How do films and coatings keep ultraviolet rays out of a house and reduce solar gain while also keeping inside heat from escaping? Experts say they act like a mirror reflecting heat back to the source. So harmful rays bounce off the outside in the summer and escaping warm air bounces back into the house in the winter.

How the window is assembled is also important to the performance. For example, the highest rated of Loewen's windows has triple glazing with two layers of low-emission coating and two layers filled with argon gas. Additionally, any exterior metal is deliberately kept away from the interior window to reduce the potential of any heat transfer. Loewen estimates the R-value of this window to be 7.9.

Noise reduction is also a feature that most manufacturers are either adding to triple gazed windows or include as distinct products in the lineup.

Millgard's Quietline windows, for example, let in 30 percent less noise than their other windows.

Meanwhile, new building codes in Florida require new windows to be able to withstand the impact of wind borne debris and has become another addition to most product lines.

Frames on the exterior typically can be aluminum, vinyl wood or fiberglass. In the last few years most manufacturers have developed a wood composite product that lasts longer than typical wood.

While windows typically last for 20 years or more, signs that a window might need to be replaced include difficulty opening and close, air leaking around the window, and condensation or fogging on or between the glass panes. Also, look for chipping, deterioration of the frame or water stands around the window.

Companies offering replacement windows usually carry lines from major manufacturers. If the frame of the existing window is in good shape, replacement windows often can be slipped into the existing space. According to Pella windows typical residential windows can cost $600 to $1,000 per window to replace. A basic window replacement product often can take less than an hour to install.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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