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"  Windows are complex. A new model may consist of wood, metal, plastic, glass, mineral oxides and perhaps an inert gas. Yet even a well-insulated unit delivers an R-value no better than an uninsulated wood-frame wall&amp;#039;s (roughly R-4 to R-5). And a house can lose nearly 12,000 Btu per hour, or about 10 percent of a heating system&amp;#039;s output, just through these leaky openings. That means there&amp;#039;s a lot at stake when it comes to choosing, installing and sealing windows. Adding storm windows cuts heating losses from drafty windows in half, and replacing them with an insulated product pares the losses still further. Finally, a modern unit holds in heat, reflecting it from you and your heating system back into the room. It seals out drafts and fends off summer sun. Here&amp;#039;s what you need to know.  Anatomy of a Draft   In winter, the inner surface of window glass can be 5 to 20 F cooler than the adjacent indoor air, depending on the window&amp;#039;s thermal efficiency and how cold it is outside. Indoor air cools as it passes over the window and sinks, producing an uncomfortable drafty zone. Insulated glass with a low U-value is warmer and improves comfort near windows. Also, the larger the glass surface, the more it needs a low-E coating, a microscopically thin layer of metal oxide that reflects back infrared energy. Without the coating, the heat flowing out from your body would flow to and through the glass, increasing your discomfort. But a low-E coating bounces the infrared energy back, so you feel warmer.  No Pane, No Gain    The tendency of a window to let heat pass is described as its U-factor--lower is more energy-efficient. It&amp;#039;s the inverse of R-value, which describes insulation&amp;#039;s resistance to heat flow; higher is better. Adding a second piece of glass dramatically improves the window&amp;#039;s U-factor. While a single-pane window is rated at 1.1, a second pane cuts the U to the range of 0.47 to 0.63. Filling the space between the panes with argon instead of air drops it to 0.37. Argon is a better insulator than air, a distinct advantage of insulated glass compared to a storm window installed on the outside of the house.  Hole-in-the-Wall Gang   Take a leaky double-hung window, with each sash measuring 35 x 26 in.--a pretty good-size window, in other words. With a gap around both sashes and at the parting rail between them, you have the equivalent of a significant hole in the wall.  Instant Gratification   Few window upgrades are as easy and useful as sealing out drafts with peel-and-stick weatherstrip. First, use spray cleaner and remove dirt and grease from the jambs or sash. When the parts are dry, cut the weatherstrip to length, peel off its plastic backing and press the strip&amp;#039;s sticky surface into place. Use V-shaped strips on window jambs; use bulb-shaped strips (shown) on the window&amp;#039;s top or bottom edge. A $5 roll of the material is enough to seal two or three typical windows.  Replacing Windows   Sure, weatherstripping helps to reduce drafts. But when larger energy savings are called for, there are three common options. New-Construction Window     // What: A new-construction window used in an existing house.  // Expect: Installation produces need for exterior and interior repairs. //Payoff: Major energy savings, good looks, tilt to clean.  // Cost: $11,000 to $15,000 for 20 windows, installedTilt-Wash Vinyl Replacement Window    // What: An insulated window installed in the existing jamb. // Expect: Quick installation. Reduced glass area due to smaller sash. //Payoff: Fewer drafts, energy savings, tilt to clean.  // Cost: $6300 to $8800 for 20 windows, installed Storm Window     // What: Sliding window in a frame; mounts on top of existing window. // Expect: Fast installation--usually one afternoon. //Payoff: Fewer drafts, improved energy savings. // Cost: $2000 to $2400 for 20 windows, DIY installation "^^<http://www.w3.org/2001/XMLSchema#string>
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