When empty nesters Tom and Joyce Conner decided to build an energy-efficient dream house in Fairfax, Va., they said they knew from experience how critical windows would be to their comfort and finances. It’s why they had replaced drafty aluminum-framed windows in their previous home a few years back.

But they were not aware of how far windows had come since then. While glass is age-old technology, ever-improving coatings, assembly methods, chemistry and design have made panes so efficient that the top-rated windows claim to guard against heat and cold as well as a standard wall.

For their new house, the Conners chose hinged windows with snug closures, triple glazing, gas between the panes, and coatings that block the ultraviolet rays that heat the house and bleach fabrics. “It’s just a superior design,” Tom Conner said. Even guests notice that the windows aren’t the run-of-the-mill double-hungs, tipped off by the black composite frames that turn and tilt to open.

But energy efficiency is not the only benefit: New windows can make a house more comfortable, quieter and secure.

The Conners’ architect, David Peabody of Peabody Architects in Alexandria, Va., specializes in “passive houses” that use little energy for heating and cooling. He offered the Conners a choice of windows made by Klearwall, H Window, INTUS Windows and Zola. They settled on Zola. “With a window like a Zola triple-glazed, the surface temp of that glass is within 4 degrees of the wall,” Peabody said. “So when you stand near those windows, it isn’t cold.”

Peabody put Zolas in his own home, not only for energy savings, but also for the quiet. “We live where the jets go by, and you don’t even hear them,” he said.

Common U.S brands such as Andersen, Marvin and Pella also make high-efficiency windows, but Peabody said that U.S. windows are more expensive than the comparable European-made models, even after shipping costs. Marvin and Pella did not respond to requests for information, and Andersen failed to provide a promised expert over a month’s period.

Before shopping for new windows, there are five things to know, and most of them can be found on stickers attached to the windows themselves or on the National Fenestration Rating Council’s website. The council is a scientific energy rating group that tests virtually all windows from reputable U.S. manufacturers.

U-factor: Most people have heard “R-values” used to say how well a product such as fiberglass insulation keeps out the cold. The higher an R-value, the better the insulation. But windows are measured by the U-factor. Contrary to the R-value, a lower U-factor number means better insulation. Some companies will assign an R-value to a window, said NFRC chairman Paul Bush, but it’s not very accurate because “R-value doesn’t have same testing protocols as U-value.”

Window makers have several ways to achieve a desirable lower U-factor. The most obvious is adding more air gaps between the indoors and the outdoors. Windows now are rarely single-pane. Most have two panes, meaning one air gap. The more efficient windows have three, and even quadruple-pane windows are available — for roughly three times the cost of a standard window.

Another way to improve the U-factor is to fill the gap between glass layers with a gas that blocks heat and cold better than plain air. Different gases work best for different-sized air gaps. The most commonly used gas is argon. It “really works its magic in half-inch spaces,” said Brad Begin, CEO of Alpen High Performance Products, a U.S. window manufacturer. In the smaller gaps of triple-pane windows, krypton gas works better.

Gas, however, has a tendency to seep out. Many window makers guarantee against leaks, with 20 years being fairly standard. “In some aspects you get what you pay for,” said the NFRC’s Bush. “Some will last 50 years easily. Some aren’t designed to last that long.” In the NRFC’s testing, windows are checked for leaks in an “accelerated weathering test,” which simulates how the window will perform over long periods of time. To pass the test, a window must retain 90 percent or more of the gas. That result is not on the sticker, but it can sometimes be found in the NFRC database or may be available on request from the manufacturer.

A third way manufacturers control U-value is through the materials used for window sashes and spacers between the frames. Window seals and spacers can be made of metal, thermoplastics, composites or foam. There is no absolute rule for which is best — let the U-factor guide you.

Solar heat gain coefficient: This measures how well a window can block sunrays that cause a house to warm — especially important in warm climates. Lower numbers mean more heat is blocked.

The solar heat gain coefficient — an industry shorthand known as the SHGC — is controlled with coatings that filter out the specific wavelengths of sunlight that cause your house to heat up. Generically it’s called low-e glass, short for low emissivity. Early coatings were made of tin, but modern coatings are silver, which, of course, is more expensive. Not all silver coatings are created equal. The thickness, adjacent materials and other factors change how well a coating works. Low SHGC numbers are better.

Air leakage: This tells you how drafty a window may be.

The frame and seal are primarily responsible for keeping out drafts. Classic double-hung windows don’t tend to do as well at sealing out leaks. Windows that don’t open at all do best, followed by hinged windows, then single sliders. (You can get high-efficiency single sliders made to look like double-hung windows.) Lower air leakage numbers are better.

Visible transmittance: This tells you how much visible light comes through the window. Higher numbers mean more light.

This is a tricky number. While you want clear glass that lets in lots of light, those low-e coatings that filter heating rays also reduce visible light. Go for as much visible transmittance as you can get at the U-factor you need.

Condensation resistance: Manufacturers aren’t required to list condensation resistance, but some do. The higher the number, the less likely the windows are to have water collect on the inside. Some windows with high condensation resistance will collect water on the outside, but that’s fine. It’s a sign that the barrier is working.

There are properties that you won’t find on the sticker that are available in custom-made windows, which is what most of the European windows sold here are. For instance, impact resistance.

Sven Shockey, design director at Smithgroup, which designed the wavy glass-fronted D.C. Water headquarters, didn’t like the security grates on his Capitol Hill home. So he removed them and put in impact-resistant windows. “Even if you hit it with a baseball bat, it would be hard to break it,” he said.

As much as glass has improved, engineers and scientists are at work on the next breakthrough. The Department of Energy’s Lawrence Berkeley National Laboratory in Berkeley, Calif., is developing what it calls a “Super Window” that could offer twice the insulation as 99 percent of the windows on sale today. The lab estimated that $20 billion of energy leaks out of windows each winter, even though double-pane windows are in the majority of U.S. buildings.

The trick is adding inner panes of very thin, durable glass, the kind made for phone and TV displays. Typically, multi-pane windows have inner panes of a thin film that has to be heated for hours to shrink it into a tight, unwrinkled layer. The new all-glass windows save that energy expense. The thin glass has come down in price as demand has increased. “We can make windows far faster at far less expense with thin glass,” said Begin, CEO of Alpen, one of two manufacturers working with Berkeley Labs to bring Super Windows to market. Andersen is the other.

The thin glass windows can be built on standard thickness frames, so the industry doesn’t have to do major retooling, and consumers won’t be faced with thick inelegant window frames.

For most people, it’s sufficient to figure out the best combination of features and put that window throughout their house. For maximum effect in a home like the Conners’ low-energy passive house, various window features are used in various locations. Windows on a sunny side have a higher SHGC than those on a shady sides, for instance.

If energy savings are the main reason for buying new windows, be careful not to overdo, said Tom Conner, who priced both triple and quadruple pane windows.

“You can’t argue with triple-pane and the payback over time on those. I think I’ve read it’s something like eight to 10 years,” he said.

The quadruple-pane, he added, costs a “ton of money. You don’t want something that is an 80-year payback.”

What do replacement windows cost?

“There’s massive price variation,” said Kevin Brasler, executive editor of Washington Consumers’ Checkbook, an organization which sends secret shoppers to research prices. “A lot of the variation comes from what the installer charges, we have found some installers charge twice as much for the same window.”

Brasler recommends that you get independent verification of salespeople’s claims about energy efficiency by going to the Efficient Windows Collaborative at efficientwindows.org. He also suggests that you compare companies warranties.

Washington Consumers’ Checkbook’s ratings of Washington-area installers, which considers cost and quality, are available to its members online at www.checkbook.org/washingtonpost/windows. (A membership is $28 annually, but Washington Consumers’ Checkbook is offering Post readers a free link to its ratings of area window installers good for a month.)

With more than 100 window manufacturers in the United States alone, many offering multiple lines options and customization, it’s difficult to generalize about pricing. Ben Beltran, Partner and project manager American Windows & Siding of Virginia, rated in the top 10 locally by Consumer Federation Of America, was willing to give some examples.

The most common replacement windows are double hung 32- by 54-inch models which typically range from about $650 to $1,800 per window, including a standard installation. “You can go cheaper,” Beltran said, “but I don’t think you’ll find the quality you are looking for.”

In that range, the $650 window would have an all vinyl sash, two panes of argon- filled glass. A buyer might opt for noise reduction glass for more quiet, a higher quality low-e coating, and a faux wood or paintable interior surface, which would raise the price to about $735 per window.

To upgrade to a window that wraps composite in real wood, the price jumps to about $950 per opening, without noise reduction. Laminating the glass will make it quieter and more impact resistant but increases cost to about $1,250 per window installed.

A mid-range window from a large name brand manufacturer — typically constructed of a composite core wrapped wood, with exterior vinyl or aluminum cladding — would cost about $1,800 each, installed.

The trick to getting the most for your money is to figure out what features you want then to shop around — much like you’d pick car, choose options, then compare dealer offers. “You have to get bids,” Brasler said. “People will say crazy things, their technology is elite, and used by NASA, and whatever, but don’t believe it.”