The age and condition of windows should be part of any home-buying decision.

If the windows are new and fit in with the look and age of the house, then check it off your list of concerns and move on.

If the windows are old and need to be refurbished to make them work as they were designed or need to be replaced with more energy-efficient models, then you’ll need to figure those costs into the equation.

If the house has a few windows, replacing them might not be a budget-breaker. If you live in a house with 25 or more windows of various sizes, expect money to be tight for a while.

How expensive? A recent estimate for replacing five double-hung wood windows with new wood windows in a 90-year-old house was $2,500, or $500 per window, including labor.

One problem with replacing windows in older houses is that they are often odd-sized. Before World War II, there was no universal construction standard, so builders made windows in sizes unique to that house.

Replacing the old windows requires often-expensive customization.

In addition, when replacing a window, you have to consider how the new window fits the overall appearance of the house.

Sometimes, the decision to replace windows in an older house doesn't rest solely with the homeowner. In historic districts, there are rules governing replacement windows.

Whatever changes you make to the exterior of your house are governed by those rules, and even if the windows you want are more efficient than what you have now, you have to comply with what the historic commission wants.

In new construction, standard windows are not that expensive compared with the total cost of the house — about 3 percent of the total cost. However, special windows - transoms, half-moons and Palladian - add a considerable amount to the cost. For a $300,000, 3,200-square-foot house, that means $9,000. Divide that by 25 standard windows, and you are talking $360 a window. Vinyl windows - which are common in new construction - are less expensive than wood ones. The price difference between the two is about $60. But price should not be the sole deciding factor.

A lot of people who want replacement windows really don't know what the windows are supposed to do, or aren't really up on the latest advances in energy efficiency.

Efficient windows are much more widely available than they were a decade ago, thanks to the rapid growth of new technologies. When buying a new or old house, how do you tell whether the windows are sound or will need to be replaced? The typical home inspector checks each window in the house to see whether it operates as it was designed.

Sometimes windows in new construction have been installed too tightly, so they pinch the screens, which won't open and close easily. In older houses, the top sash often has been painted shut because people rarely move it up and down.

Windows in very old or historic houses usually operate surprisingly well. A lot of old house owners like living in them, and they have maintained the windows. To make older windows more efficient, homeowners strip and paint each sash, replace the glass and hardware, and install new weights and chains so that they move up and down easily.

To compensate for their inefficiency, they seal gaps between the moving parts with weather stripping, or fill the gaps between the window frame and the wall with caulk.

Storm windows are often installed to help reduce heat loss, and there are temporary fixes, such as plastic film kits that create the effect of an interior storm window.

How do you find leaks? Wet your fingertips, and run them around the window frame to feel a draft.

According to the nonprofit Alliance to Save Energy in Washington, replacing windows adds value to a house in several ways.

It improves the appearance and resale value of the home. It reduces maintenance costs such as painting, and makes cleaning easier with tilt-sash designs.

It improves comfort by making windows feel warmer in winter, or by cutting down unwanted solar heat in summer, and it reduces damage to furnishings by blocking ultraviolet light that can damage fabrics and other materials.

According to data collected by Remodeling and Realtor magazines, every dollar will return 96 cents on that investment if the house is sold within a year of completing the work. That is a national figure, however. The actual return could be much higher, depending on the area in which you live.

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