A bay window actually is composed of three windows joined, or mulled, at the factory to make a single large unit. The wide center window is flanked by narrower casement or double-hung windows. Two vertical uprights, called mullion posts, separate the three. The most common style is the angled bay window, which protrudes from the house and slants back toward the wall at a 30- or 45-degree angle. A box bay is square-the side sash come straight off the house at 90 degrees. A box bay with a glass roof is known as a garden bay or greenhouse window. All major window manufacturers offer angled bay windows in both 30- and 45-degree configurations; a few make 60-degree bays. Most also offer 90-degree box bays. Size. Each bay type comes in hundreds of standard and custom sizes to fit any opening. Standard sizes typically range in width from 3 feet 6 inches to 10 feet 6 inches, and in height from 3 feet to 6 feet 6 in. Materials. Most windows are made of wood or vinyl. Exterior finish options include primed wood, extruded vinyl and wood clad in low-maintenance aluminum or vinyl. Primed-wood windows are economical, but they must be painted and periodically scraped and repainted to prevent rot. Vinyl windows are also affordable and maintenance-free, but their frames include vinyl on interior surfaces — a look not everyone likes. Clad windows usually cost more, but offer a low-maintenance vinyl or aluminum exterior and a handsome all-wood interior. Most bays come with insulated glazing-two panes separated by an air space. If you want higher energy efficiency, order the window with argon gas between the panes, or opt for low-e glazing that admits solar heat in winter and reflects it in summer. Homes in cold-weather climates might benefit from triple glazing (three panes, two air spaces), which provides 15 to 25 percent higher energy efficiency than standard insulated glazing. What you'll pay. Although prices vary widely depending on size and window construction, expect to pay between $800 and $1,100 for a 3-feet-high by 6-feet-wide vinyl-clad 30-degree casement bay window. A custom-made unit will cost at least 15 to 20 percent more and take four to six weeks for delivery, compared with less than a week for most standard-size bays. That's why it's important to shop around for the window that best suits your home, budget, and time schedule.
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