Home window buying guide

Manufacturers mark their replacement windows with a U-factor, a measure of a window's ability to conduct heat. The U-factor is the reverse of the R-value, which is a measure of insulating ability. The lower the U-factor or the higher the R-value, the better a window can keep your home cool in summer and warm in winter. The R-value may be better known to the general public, but manufacturers avoid listing it because it might seem less impressive. The R-value of the very best windows is about 2 or 3, equivalent to that of an uninsulated wall. The solar heat gain coefficient is shown as a fraction and indicates how much of the sunlight that hits a window makes it inside as heat. For cold climates, look for the highest SHGC number you can find; in warm climates, 0.40 or less; temperate areas, 0.55 or less. Visible transmittance measures how much visible light a window lets in. The higher the VT, the better.

Even the best windows won't deliver the look, comfort, or savings you expect if they're installed poorly. Many major window manufacturers, including Andersen, Marvin, and Pella, train and certify installers for their specific products. Using the same contractor for purchase and installation can minimize the chances of problems arising later. Readers who used an installer recommended by the window manufacturer were more satisfied overall than those who used contractors employed or recommended by Home Depot or Lowe's, according to our Home Improvements Survey. Also look for certification from the American Window & Door Institute (www.awdi.com).

Before work begins, read the installation instructions for the windows you've chosen online, and check that the installer plans to follow those instructions, right down to details such as type, amount, and placement of flashing and insulation. Deviating from the manufacturer's recommendations could void the warranty. If you plan to paint the windows, have the installer use acrylic-latex caulk, which can be painted, not silicone.

These windows were our top overall scorers. On wood-frame windows, the wood is clad in vinyl or aluminum for durability. The fiberglass-frame windows are all-fiberglass. The major brand wood- and fiberglass-frame windows we tested excelled at keeping out cold air and rain when they were new. After we subjected each window to a week's worth of extreme temperature swings that that made their components flex, expand, and contract, the best models showed little or no loss of performance.

Although relatively inexpensive and maintenance-free, vinyl windows tend to leak air a bit more. Also, they lack the visual appeal of wood, and they can't be painted or stained, so they may be inappropriate for an older home.

They're hinged at the top and open outward. They leak less air than sliders and single- or double-hung windows because the sash presses against the frame to close. They also offer better ventilation than sliders of the same size and can be left open when it's raining because they deflect rain during storms. But screens can be placed only on the inside.

They're hinged at one side, like a door, and usually open outward. Like other hinged windows, you typically get less air leakage because the sash presses against the frame to close. They're easy to clean and also offer better ventilation than sliders of the same size because they open to the full glass area. You can position them to catch passing breezes. Two drawbacks are that screens can be placed only on the inside and most open using cranks that must be operated manually.

These are used where lighting but not ventilation is important. Pros: These windows are airtight and are available with decorative glass accents or in unusual shapes. But fixed windows don't open, so they provide no ventilation.

The opposite of awning windows, they're hinged at the bottom and can open either inward or outward. They're often installed above a door or another window, protected by eaves. You get less air leakage than with sliding and single- or double-hung windows because the sash presses against the frame to close. But screens can be placed only on one side.

Double-glazed windows have a sealed space between the two panes of glass, and the air in the space provides an added layer of insulation. Compared with a single pane, double glazing can cut heat loss nearly in half. The insulating value of triple-glazed windows is higher still, but the extra layer adds to weight and cost.

Clear glass allows large amounts of radiant energy to pass through--heat in from the sun in summer, heat out from your house in winter. A low-E, or low emissivity, coating is a microscopically thin metallic film that acts something like a two-way mirror, reflecting heat back into the house in winter and blocking heat from the sun in summer. Which it does better depends on how the coating is applied. The windows can be fine-tuned for different climates, producing Southern or Northern windows, for example. Some coatings may darken the glass, like tinted glass in a car, an effect some people might not like. Check a sample at the store.

Instead of air in the sealed space between glass panes, these windows use argon, krypton, or other inert gas. These gases are denser than air, so they provide better insulation.

This is the vinyl or aluminum that covers the exterior of a wood window so that it doesn't have to be painted.

On windows with this feature, the sash (moving part of a window) can be tilted for easy cleaning.

Alside is a manufacturer and marketer of vinyl windows that are available in multiple replacement and new-construction lines in popular double-hung and casement styles and feature Low-E and argon-filled glass for high efficiency. Alside windows are primarily sold through distributors and are mid-priced.

Anderson is one of the leading manufacturer and marketers of windows. Andersen windows are available in multiple replacement and new-construction lines in popular double-hung and casement styles. Anderson window lines include wood, clad, vinyl, and composite construction and low-E and argon-filled glass for high efficiency. Anderson markets a line of stock sizes and has extensive special-order and custom options. Andersen windows are widely available through independent home centers, dealers, and Home Depot. Anderson markets a line of vinyl windows under the American Craftsman brand at The Home Depot and a composite line under the Renewal by Anderson name through certified installers.

Certainteed is one of the leading manufacturers and marketers of vinyl and composite windows. Its windows are available in multiple replacement and new-construction lines in popular double-hung and casement styles and feature Low-E and argon-filled glass for high efficiency. Certainteed windows are primarily sold through distributors and dealers and are mid-priced.

Jeld-Wen is one of the leading manufacturer and marketers of windows. Jeld-Wen markets windows under the Jeld-Wen, Pozzi, Caradco, and Wenco brand names that are available in multiple replacement and new-construction lines in popular double-hung and casement styles. Jeld-Wen window lines include wood, clad, vinyl, and aluminum construction and low-E and argon-filled glass for high efficiency. Jeld-Wen made windows are primarily sold through independent home centers and dealers.

One of the leading manufacturer and marketers, Marvin windows are available in multiple replacement and new-construction lines in popular double-hung and casement styles. Marvin window lines include wood, clad, and composite construction and low-E and argon-filled glass for high efficiency. Marvin markets a line of stock sizes and has extensive special-order and custom options. Marvin windows are sold through independent home centers and dealers and are premium priced.

A leading manufacturer and marketer of windows, Peachtree markets windows under the Peachtree, Crestline, and Weathershield brand names that are available in multiple replacement and new-construction lines in popular double-hung and casement styles. Peachtree made window lines include wood, clad, vinyl, and composite construction and low-E and argon-filled glass for high efficiency. Peachtree windows are primarily sold through independent home centers and dealers and are premium priced.

One of the leading manufacturer and marketers, Pella windows are available in multiple replacement and new construction lines in popular double-hung and casement styles. Pella window lines include wood, clad, vinyl, and aluminum construction and low-E and argon-filled glass for high efficiency. Pella markets a line of stock sizes and has extensive special-order and custom options. Pella windows are widely available through Pella Company owned stores, dealers, independent home centers, and Lowes. Pella markets a line of windows under the ThermaStar by Pella name at Lowes.

Survivor is a manufacturer and marketer of vinyl windows. Survivor vinyl windows are available in multiple replacement and new-construction lines in popular double-hung and casement styles and feature Low-E and argon-filled glass for high efficiency. Survivor windows are sold through Lowes and distributors and are low priced.