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Modern home with large windows. iStock

Windows play a critical role in the three most important aspects of home design. The first is appearance. Next to the shape of a house (Colonial, ranch, Cape Cod), windows are the most significant factor influencing how the place looks to the outside world. The second element is site embrace. Windows capture views and make the connection between the indoors and the natural world beyond. Last, windows are about comfort. They let in light and air and protect against extremes of weather.

How to decide where to place a window

The power of windows is often most evident when the size, type, or location is miscalculated. Then rooms are either glaringly bright, depressingly dim, or stultifyingly stale. But if planned right, windows can be the key to enjoying your home's ambience, inside and out. Here's how to think about them.

3 Rules for Choosing Windows

1. Don't Be Bound by Tradition

Most people think of their houses — and the windows that go in them — as representative of an iconic "type," like Colonial or Queen Anne. But it's important to remember that virtually all traditional house styles have window designs that originated at a time when walls were not insulated, there was no central heat, and glass didn't come in pieces bigger than a dinner plate. Early builders didn't forgo picture windows by choice; the technology simply wasn't available.

Does that mean that only modern houses can feature large expanses of glass? No. It's not wrong to think about big windows facing wonderful views in even the most historically "correct" houses — you just have to pick your spots wisely. In most traditional homes, that means away from the street-facing facade. (Fortunately, many street-scapes aren't worth bringing inside.)

2. The Front Doesn't Have to Determine All the Other Sides

The front of a house should be friendly to visitors and convey a sense of the home's inhabitants. For many people that means a traditional, symmetrical approach, especially in a neighborhood where existing homes set a style you would like to respect (or is mandated by code). But the pattern of windows on the front of your house doesn't have to be repeated on all the other sides. If your windows are consistent in the way they are treated — basic type, grille patterns, and trim — they can handle great variations in quantity and style.

The big mistake is to treat windows so differently that something is clearly "wrong" — say, a plate-glass window set directly adjacent to a double-hung eight-over-eight Colonial window. If you give that outsized window its own wall, and keep the trim and muntins simpatico with those in the rest of the house, you can enjoy your big view without compromising the integrity of your home's design.

3. Keep Comfort in Mind

What works best on the outside to give your house a sense of scale and visual identity isn't always ideal on the inside, where windows should respond to how rooms are used and the orientation of the house to the sun and wind. For instance, windows facing east and west accept the very low angle of spring and fall sunlight, which can often be blinding — especially troublesome in a room used for watching television or working on a computer.

For windows on those walls, you will need shades or curtains, or to set the sill more than 4 feet off the floor to reduce glare. Similarly, if you know in which direction the prevailing wind blows, you can increase the amount of operable glass in that area, allowing for more passive ventilation and cutting down on air-conditioning bills.

Today's windows are far superior to their older counterparts in terms of blocking unwanted drafts, but you still need to take into account radiant heating and cooling. No matter how well insulated, large panes of glass will suck heat in winter and invite it in during summer. For maximum comfort in cold climates, it may be necessary to have large areas of glass directly washed by a heat source, such as convective heat from a radiator or blown heat from a forced-air system (just know that this will raise your heating bill).

On the flip side, the best way to guard against heat gain in warm weather, especially with south-facing windows, is to shade the glass with long overhangs so that the high angle of the sun during summer cannot penetrate deeply into your room. Curtains and shades are another defense.

Do's and Don'ts for Mixing Window Styles

When adding new windows to an existing wall, the trick is to keep the treatment consistent, with compatible trim, grille patterns, and muntin style. A modern plate-glass window looks out of place in a perfectly symmetrical antique facade.

Match the Window to the Room

Different areas of the house have different requirements for windows. Here are a few guidelines for thinking about windows on a room-by-room basis.

Living Room Windows

The living room is typically one of the largest rooms in the house and therefore can accept the biggest windows, but that makes sense only if there's something to look at. For example, if your living room faces a tight cul-de-sac, the view of other people's cars might not be the best use of a large array of windows. Big, south-facing windows can fill all the spaces feeding off the living room with bright, ambient light; just be aware of the potential for overheating in summer. Many living rooms also contain glass doors to decks or patios. Ganged together, windows and doors can create a sense of openness and bring in the outdoors in a way that makes a home seem larger than its actual size. Integrate them by aligning the tops of the openings and using coordinating trim.

Dining Room Windows

Windows in the dining room should be thought about with a great deal of circumspection. Most dining rooms are used at night, so do you really need a giant window to look through when it's dark outside? An increasing number of homeowners are choosing to put dining spaces in areas with few or no windows, relying on art, a fireplace, or even furniture to create more valuable visual interest when the sun goes down.

Family Room Windows

Given the variety of activities that go on in a family room, window placement can be a challenge. For instance, high light levels and low sun angles interfere with TV viewing — and if you've invested a lot of money in a plasma screen, you certainly want to be able to watch it. So if the view from your family room is fantastic, you may want to opt for big windows and relocate the TV and computer. If homework or other desk work is to be done in the room, think about which side you'd like the natural light to come from: For right-handed people, light from the left is best; vice versa for lefties.

Kitchen Windows

Double-hung windows over sinks are tough to open and close; casements are the best choice when you have to reach over a sink or counter to get at them. Windows and cabinets often compete for wall space in kitchens. For the best of both worlds, create a walk-in pantry for storage and have larger windows over the counters, where you tend to spend more of your time.

Bedroom Windows

While bedroom windows admit light and views, their most important function is to allow for sufficient air flow on warm nights. Casements give the most operable area, awning windows can remain open even in light rain, and tall double-hungs can have both the upper and lower sash opened. Bedroom windows are also subject to some safety considerations: If you have a second-floor bedroom and don't have a back staircase, building codes require that the windows be of a certain size and sill height to allow you to escape and firefighters to get in.

Bathroom Windows

Privacy is key. Transom windows, skylights, or even glass block can give you a fair amount of ambient light without compromising privacy. However, it's a real benefit in most bathrooms to have operable windows, rather than relying solely on a fan.

Match the Window Height to the Space

How tall is a standard window?

The vast majority of windows in American houses have their heads set to the standard door height of 6 feet 8 inches. With a standard wall height of 8 feet, that leaves a 16-inch gap between the top of the window and the ceiling — perfectly sized for standard headers and plates, and accommodating of even the biggest window trims and cornice moldings. But in a room deeper than 12 feet, that band of wall pinches the view, and it's silly to keep the top of the window at 6 feet 8 inches with newer 9- and 10-foot ceilings. Don't be afraid to set the top of the window clear up to the cornice trim. In fact, the cornice can even function as the window's head trim, if you plan properly.

What is the standard window height from the floor?

There are a few structural issues to consider when raising window height. Set a window closer than 10 inches from the ceiling and odds are you'll have to raise the header up into the structure of the floor cavity or rafter space above (called an "upset" header). That type of construction costs extra, especially in an existing home, but the additional head height can make all the difference in a large room.

Similarly, many windows can be set lower than the common height of 3 feet above the floor. Traditionally, that height was meant to allow the placement of furniture under the sill. But if nothing is in front of the window, lowering the sill will increase ventilation (the more operable glass, the greater the air flow) and allow you to bring in more of a great view. Note these safety considerations, though: In a child's bedroom, a windowsill lower than 2 feet will need a window guard, and any window with glazing lower than 18 inches off the floor must be safety glass.