Thirty years after the first double-pane window, we are finally catching up with the rest of the world. Here’s a look at where we’ve been and where we’re going with window technology.
A Little History
Before 1970 most windows were made of wood or aluminum and had a single pane of glass. Single-glazed windows, as they are called, had a thermal resistance of R-1. They leaked air, created condensation, and transmitted lots of heat and cold.
After that time, double-glazed windows were born, with two panes of glass and an air space between them which produced an R-value of about 2.0 (or U-value of .50). They were a bit warmer and quieter, but still condensed easily and leaked around the edges. When gas such as Argon was later added to the air space along with better gaskets and thermal breaks, condensation came under control, as did air leakage, which raised the efficiency higher to about R-2.5 (U-value of .40).
In the late ’70s, a revolutionary coating was invented that reflected certain wave lengths of light and allowed other wave lengths to pass through it. It reflected infrared energy but visible light waves were transmitted. This low-emissivity coating eventually became known as low-e for short. The addition of this coating boosted R-values to 3.0 (U-0.33).
In the early 90s, windows were made with two or three coatings of low-e, which increased R-values even higher. They blocked other spectrums of light such as UV while allowing more visible light to pass through them. These high performing windows blocked 99 percent of the UV light, which reduces solar heat gain. The net result was that the glass stayed warmer on the inside during winter and blocked excessive heat in the summer.
Energy Star Ratings
In 2010, the U.S. Environmental Protection Agency’s ENERGY STAR Program made some game-changing rules about how they define energy efficient windows. These newly rated windows make reductions in your energy bill, lower demands on the power grid, and reduce global warming. But is this enough? Compared to European standards, for example, the answer is a big no. U.S. energy standards fall far short. Rather than wait, however, some innovative manufacturers are moving forward—way forward, and are raising the bar on window technology.
High Performance Windows
Last year, the Empire State Building used high-performance windows to retrofit 6,514 old windows, which reduced energy costs by 38 percent and will pay for themselves in three years. The Passive Haus in Germany used them to help create a net-zero energy home, and the new MUM Sustainability Learning Center in Fairfield used them in their new LEED Platinum-rated building, which is estimated to reduce energy costs by 30 to 50 percent.
This new window technology integrates a variety of energy efficient improvements such as foam-filled fiberglass frames, ultra-low conductance krypton and xenon gas filling, and super-insulating edge spacers between the glass. These features reduce heat loss and air infiltration by more than 50 percent over current Energy Star standards. But the quiet, invisible performer is the high-tech super-insulated glazing.
This new glazing, which was, by the way, invented in the ’70s, uses a thin film of plastic suspended between two panes of glass. The film, in effect, creates a triple glazing and provides two more surfaces for low-e coatings. Today manufacturers have broken the mold by adding up to two or three pieces of film between the glass, which produces an incredible full-frame rating of R-7, R9, and up to R-11 (U-0.09), exceeding Energy Star standards by a factor of almost 4 to 1.
What are the Benefits?
Initially, high-performance windows are more expensive, but they pay for themselves quickly, especially with rising energy costs. Your house will stay warmer during the winter and cooler in the summer, resulting in 20 to 40 percent savings on your energy bill.
In addition, their multiple glazing and insulated frames reduce outside noise, creating a quieter home. The fiberglass frames are durable and built to last a long time. You’ll have less air leakage and less dirt, pollen, and other pollutants entering into your home. High-performance windows stay drier in the winter, meaning less condensation and less mold and mildew. They also block 99 percent of UV light, so there’s reduced interior fading. Finally, they improve your home’s resale value.
Replacing Older Windows
If you don’t know the age of your windows, look in the lower corner of a double pane window to see the manufacturer and date stamp on the glass.
It’s definitely a good idea to replace windows prior to 1990 with at least Energy Star rated windows or better. High performance windows are expensive but will provide the best bang for your buck. Buying the best you can afford now will save you money in the long run, especially as energy prices and window prices climb. Avoid the temptation to buy the least expensive vinyl windows, which may not hold up after several years of duty.
If you can’t replace all of your windows at once, start with just a few and add more each year. Consider it like a forced savings plan that will pay dividends in the future.
Find out more about Energy Star certified rebates from your local utility or cooperative.
©2011 Joel Hirshberg. Joel is the founder of Green Building Supply in Fairfield.
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