Today's record high cost of fuel and energy, coupled with the almost prohibitive cost of new housing, has caused many people to update their present home, rather than buy a newer and perhaps more fuel-efficient house that will cost less to run.

One type of improvement that has become increasingly popular in recent years is replacing old, drafty windows with prefabricated, better insulated units that not only contribute greatly toward lowering heating and air-conditioning costs, but also add to the aesthetic appearance of the home and help lower future maintenance costs by eliminating, or at least greatly minimizing, the need for painting.

Although conventional stock-size wood or metal windows designed for use in new construction can just as well be used to replace existing residential windows, often it is hard to find windows the exact size needed to fit the openings left when the old ones are removed. As a result, there may be considerable additional carpentry work (and expense) involved in framing in the old opening to make it smaller, or in cutting structural members to make the opening larger.

Because most homeowners are reluctant to go through this kind of messy alteration job, an entirely new line of replacement windows has been introduced in recent years. These are sold by dealers-installers who specialize in replacing old windows with attractive, prefabricated, factory-assembled windows specifically designed for the replacement market.

After measuring the existing openings, new windows are fabricated to the exact size required, and the new windows are installed almost completely from the inside of the house. In many cases the changeover takes no more than an hour or two for each window.

The new windows come completely assembled with permanent weather stripping, and insulating glass (double or triple glazing) that does a good job of keeping heat in during the winter and out during the summer. Screens are also available. Their airtight construction and snug-fitting assembly eliminates drafts and prevents heat loss through cold-air infiltration, especially when the wind is blowing, thus further contributing to fuel savings. e

Until recently, the replacement windows sold in this country were almost always made of aluminum with either a baked-on enamel or vinyl coating that would last for years without needing to be painted. Early models caused problems, however, because aluminium is a good conductor of heat. This meant the metal frames became cold on the inside every time it was cold on the outside. As a result there was not only a continuous loss of heat during the winter (and a waste of energy during the summer when air conditioners are running), there was also a serious condensation problem on the inside with water runnig down and staining or rotting sills and plaster.

Today, however, all better quality metal windows have a built-in thermal barrier, a plastic spline or divider that physically separates the inner and outer parts of the metal window frames so that one side cannot conduct heat (or cold) to the other side. This greatly minimizes the condensation problem, though it still cannot eliminate it entirely. (When it gets cold outside and the humidity inside is above a certain moderate level, condensation may still occur -- because some heat still passes through the air spaces inside the frame and chills the inner metal in spite of the thermal barrier.)

Wood windows do not have a problem with condensation because wood is a good insulating material, but wood windows come in stock sizes that do not always match. In addition, most wood windows have to be painted periodically -- a chore most homeowners would like to do without.

Some manufacturers now produce wood windows with a vinyl sheathing that eliminates the need for painting. And one large manufacturer of prefabricated wood windows (Andersen Corp.) is also marketing a special "customizing kit" of vinyl-clad filler pieces that make it possible to fit stock-size windows into existing openings with only a little carpentry work, using the strips to fill in the spaces around the new window.

The newest type of replacement window is the solid vinyl window. Now being produced by a number of manufacturers who produce the extrusions and parts for dealers who then assemble and install them, the all-vinyl window offers several advantages that appeal to homeowners:

1. Since they are made of solid vinyl the color goes all the way through and there is no finish to wear off or chip and fade; thus they will never need painting, inside or out.

2. Vinyl is a good insulator and does not conduct heat, so the windows are more energy-efficient than metal and no thermal barrier is required. The large air chambers inside the frame, combined with the insulating qualities of the vinyl itself, serve as a barrier to heat transfer from inside to outside or vice versa, thus saving on both heating and air-conditioning costs.

3. Because the vinyl does not get as cold as a metal frame will, there is little or no problem with condensation under normal winter conditions.

4. There is never a problem with rotting corrosion, oxidation and pitting as can occur with metal -- even where the windows are exposed to salt air or heavy industrial fumes. The vinyl can be easily scrubbed clean.

5. A good quality vinyl window frame won't dent or bend.

6. Because vinyl windows are self-lubricating, they open and close easily without sticking or binding.

Like all better quality replacement windows, regardless of what they are made of, vinyl replacement windows come in various styles, including fixed picture windows and horizontal sliding or rolling windows. However, the conventional double-hung style with an upper and lower sliding sash is the most popular.

Almost all have double-thick insulating glass that is permanently sealed into each sash frame, and in many cases tripple glazing and can be installed for further energy savings (worthwhile in very cold areas). Sliding or removable custom-fitted screens are usually an optional extra.

The windows also have built-in tensions balances or spring-balance systems that are virtually maintenance-free, and almost all have another important advantage every homeowner will appreciate: The sash tilts inward, or can even be easily removed from the inside for cleaning or for replacing a broke pane of glass.

Having replacement windows installed is costly -- a typical double-hung window measuring about 3 feet wide and 48 to 50 inches high may cost from $250 to $350 installed ($300 is closer to average), while even a small sliding or rolling window may cost over $100 installed. (These are prices for good quality vinyl windows; comparable quality aluminum windows will cost about the same or slightly less in most areas.)

Once the windows are installed, savings will help recoup some of this cost, starting almost immediately -- in fuel savings in the winter, and lowered electric bills during the air-conditioning season, but it will take a few years to save enough fuel and electricity to fully repay the homeowner for the total cost of having new windows installed.

There will also be money and time saved in not having to paint your windows anymore, and by eliminating cold drafts most homeowners will find they can be comfortable with lower thermostat settings -- thus further saving on fuel bills.