Which Windows Do What? Knowing what you need a new window to do will make deciding on the appropriate size and style easier. There are four primary reasons for changing the look or location of your windows. ADDING LIGHT
Even the most sophisticated artificial lighting can't substitute for natural light. A quick rule for estimating the size of new windows: Useful light penetrates two and half times the height of the opening. Clerestories — a band of windows running under the roofline along the top of a wall — are an exception. Because of their lofty location, they tend to funnel light deeper into a space. Also remember that the amount, angle, and quality of sunlight change during the year, and the difference can be substantial. East- and west-facing windows admit the warm glow of dawn and dusk, but they also bring the hard, glaring beams of afternoon light. A southern exposure is the best source of warm ambient light, while a northern opening catches cool, indirect light. An architect can help you forecast how the sun will fill the room and also give you a sense of how light will play through your house by building a scale model and facing it toward the sun.
The color of the walls surrounding the window is another variable: Lighter tones reflect the sun, brightening the room, while darker ones absorb its rays. Shade trees should also be factored into the picture; new windows aren't an enlightened move if they're blocked by towering evergreens. COOLING DOWN

Some windows are especially effective if your wish list includes better ventilation. Crank-open casement models are best for regulating and directing air currents. Awning varieties, which pivot upward, also shield interiors from rain. When planning for improved ventilation, note the seasonal wind patterns around your property and try to pair up windows on opposite walls to capture the cross ventilation. Maximize airflow by locating windows away from corners. And remember, the velocity and cooling power of cross ventilation increase if the breeze enters through a smaller window and exit through a larger one. FRAMING A VIEW
It's tempting to use sweeping picture windows to highlight a striking vista. Unfortunately, really big expanses of glass cost really big money. And integrating them into an existing wall often involves a formidable structural challenge — and even more cash. Instead, consider ganging several smaller windows together. Besides being a potential money-saver, the windows divide the view into components that can be taken in individually or as a whole. "The human eye can see only about 60 degrees," says architect John Allegretti, of Michigan City Associates in Michigan City, Michigan. "Segmenting views makes them more interesting and manageable." DEFINING A PLACE
With their three-dimensional qualities, bay and bow windows are especially adept at defining a work space, breakfast area and other places. Windows that are flush with the wall can also highlight an area. For maximum impact, cluster them in a focal point or put a single fixed pane in a prominent place. Explore your home for opportunities to add light and interest. An awkward blank wall at the end of a hall or on the stair landing could be the perfect spot for an accent window.
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