Seeing the Big Picture on Window Choices : Among factors for shoppers to consider are size, style, design and energy efficiency.

Of the many components of a home, windows work about the hardest because of the multitude of functions they serve.

They let in light, provide ventilation and a view, and are major elements in interior and exterior design. And they do all that while saving on heating and cooling costs.

At one time, a homeowner couldn’t find multifunctional windows all wrapped up in one package. The standard window was a wood, single-pane affair that let in not only light but also the cold.

Then came some slight alterations in materials and design. Wood windows were joined by aluminum ones. Picture windows became a trend. But single panes were still the industry standard, and the cold kept finding its way home.

Since then, advances in technology and an expanding aesthetic sense among home designers and owners have prompted window manufacturers to introduce sizes, styles and designs for updating or restoring the appearance of a home while being energy-efficient.

Since windows can account for about a third of the heat lost from a house, efficiency should be a prime consideration when shopping for them.

The standard industry issue today is a double-glazed window consisting of a layer of air hermetically sandwiched between two panes of glass. The air serves as an insulator to thwart the escape of heat from the house.

Measure of Energy Loss

How effectively a window does that is gauged by its U-value, a measure of the amount of heat transmitted through all the materials in a window, including the frame. The lower the U-value the better. A single-pane window has a U-value of about 1.05, while the U-value of double-glazed windows on the market ranges from about .47 to about .52, meaning that double-glazing loses about half as much heat, or air conditioning, as single glazing.

Some manufacturers also offer triple-glazed windows that contain three panes of glass and provide greater energy-efficiency. However, they are considerably heavier and cost more and, thus, are used less frequently.

About the same effectiveness can be achieved with Low-E glass. Low-E stands for low-emissivity glass, which is glass coated with a transparent, heat-reflective material (the coating is applied to the inside pane of a double-pane window).

There is a technical explanation of how this makes windows more efficient, but the simple one is this: In winter, the heat inside the house is prevented from escaping, and in summer, the heat outside is kept from entering. In addition, the coating filters out ultraviolet rays from the sun that cause curtains, sofas and rugs to fade.

Types of Frames

Glazing, however, is only one part of the equation for an efficient window; the other is the frame.

Wood, which was once the sole material used in window frames, is still around, used by itself and in conjunction with other materials. By itself, it provides an authenticity in restorations of traditional, older homes and lends a warm appearance inside.

Its advantage is that it can be painted to complement or accent any exterior color scheme. Its drawback: It requires exterior maintenance. Wood windows also are the costliest.

“Clad” windows, the most common on the market, offer a compromise for those who want to avoid exterior maintenance but still want to enjoy the warmth of wood inside.

These frames are made of wood, but parts are clad in either vinyl or aluminum, giving them an exterior nearly impervious to the elements and an interior side that can be stained or painted. The wood also acts as a thermal break to help insulate the overall unit.

Colors Limited

The disadvantage: The vinyl and aluminum coverings come only in a limited number of colors. Aluminum frames, furthermore, can oxidize and become pitted when used too near a marine environment, such as along the shore.

Another option is a solid vinyl frame. Frames in the better-quality lines are constructed with multiple chambers. A cross section of one would show a number of hollow chambers where dead air acts as an insulator.

Prices can fluctuate considerably when custom sizes are required. In new construction or a major remodeling, there’s not much of a problem with size. Unless plans call for exotic window arrangements, appropriate stock sizes can be designed in.

In straight replacement, however, the size of the existing opening dictates the size of the window. Some might require custom replacements, but there are instances when stock windows can be used, even if they won’t fit exactly.

Check Building Department

In this situation, find a stock size close to but smaller than the existing opening. It is easier and less expensive to throw in a couple of extra studs to close up an opening than to tear out framing to enlarge it. Be aware, though, that some patch work will be required on the sheeting and siding, which is why a major window replacement project is best done when a house is being re-sided.

If the work involves enlarging a window opening, check with your local building department, especially if there is a readjustment of structural components such as headers. To play it safe, check local building codes to determine whether they pertain to the kind of window replacement project. There are requirements for window sizes to allow exit in emergencies.

After the serious business of window shopping--finding an energy-efficient window in the right size--comes the fun part. There are literally thousands of styles, sizes and shapes to choose from to give a home just about any look desired.

Double-hung windows, the familiar up-and-down style, give a home a traditional or “period” look. The taller, sleeker casement windows (the ones that crank out vertically) add a contemporary touch, and because the whole sash opens, a casement window permits maximum ventilation.

Awning Windows

Bay and bow windows--the expanded version of the picture window--open up the interior of a home, physically and visually. (The difference between the two, by the way, is that bows have a gentler curve to them and jut out from the house less than bays.)

Awning windows, which crank out horizontally, allow full ventilation even when it’s raining outside. Placed high up in a room, end-to-end in a horizontal band, they afford light, ventilation and privacy.

Fixed windows, since they do not open, can be made in almost limitless geometric shapes and be used alone or in combination with other styles to create unique arrangements. Currently the most popular example of this is the placement of half-circle windows above other kinds.

And even though what works in one place often doesn’t work in another, there seem to be fewer restrictions when it comes to creating window arrangements.

Distributed by the Los Angeles Times-Washington Post News Service .