Although window screens are considered almost a necessity to keep insects out of most homes and apartments, a new type of fiber glass screen mesh now available makes it possible to use these same screens to screen out a high percentage of the sun's heat that normally comes through the windows during the summer months.

Referred to as solar screening, and currently being produced by several manufacturers who market it through lumber yards, home centers and chain stores, as well as in some large hardware stores, this screen mesh is made of specially woven fiber glass yarns that are often coated with vinyl to give them added durability.

Solar screening differs from conventional insect screen mesh (metal or plastic) in that it has a much heavier and broader weave, with wide fiber glass yarns woven into a ribbed pattern that presents a reflective surface to solar rays. While it will keep insects out in the usual manner, it can also reflect away as much as 75 percent of the sun's heat that would normally be absorbed by the glass in a typical window, according to studies made by the various producers-of this material.

This means that it will substantially lower interior temperatures on sunny days, and result in lower operating costs for air conditioners. Draperies or blinds on the inside of a window have long been known to provide similar protection against heat penetration through windows, but having the reflective material on the outside is much more effective and results in substantially greater savings because the sun's heat never gets as far as the glass, and never reaches the inside.

In addition to its effectiveness as a solar reflector, this new type of fiber glass screening offers three other benefits. It cuts down substantially on glare; affords added privacy to occupants weave looks almost opaque and makes it difficult to see in; and reduces light and heat greatly to lessen fading of draperies, carpets and other furnishings.

On the other hand, from the inside the view is not obstructed to any great extent and there are no distortions - outside just looks somewhat shaded and not as bright.

Indications are that this solar screening may help to cut heating bills in the winter reducing the wind chill factor, which accounts for a great deal of heat loss through the glass. In some cases the screening will actually reflect some of the heat lost back into the window.

The screening is sold in rolls of various widths to match the widths of most standard-size windows, and it comes in a choice of shades: bronze, gray, charcoal, white and bone. It can be installed in existing screens by removing the present mesh and replacing it with the new material, or the homeowner can build completely new solar screens by using aluminum frame members speciality designed for this purpose.

To replace the mesh on aluminum frames that have conventional insect screening start by laying the screen flat and prying out the old plastic spline that holds the mesh in place. This can be done with a screwdriver, starting in the corner where a spline end is visible. If the old spline is still firm and flexible it can be reused; otherwise discard it and buy new splining material (it is sold in the same stores that sell the screening material).

Now cut a piece of the solar screening large enough to overlap the frame by about one inch on all the four sides, and place this flat on top of the frame without wrinkles or folds. Lay the spline in place on top of the mesh, directly over one of the grooves on one of the longer sides. Starting in one of the corners, force the spline into the channel with one of the special concave or round-edge rolling tools sold for this purpose. Keep pulling the screening taut as you go so that it is stretched tight.

When one side is done, stretch the screening across the frame and continue around the other sides, forcing the spline firmly into its groove as you go and continually pulling the mesh tight to avoid wrinkles. Only moderate pulling is required; pulling too hard can cause the screen frame to bow or curve.

When the four-sides have been done, check the four corners to make sure the spline is sitting firmly at the bottom of the groove, then trim the excess screen material off with a sharp knife or razor blade. Hold the blade at an angle, pointing it toward the outside of the frame as you trim, rather than straight down.

For windows that need new screens (frames and all), or for maximum solar protection, full-length screen frames can be quickly and easily made with special screen framing members made by Reynolds Metals Co., Richmond 23261.

Sold through many hardware stores, lumber yards and home centers, these frame members come in 6-foot and 8-foot lengths, and there are special corner locks or braces that made it easy to assemble the sections after they have been cut to the required lengths.

The sections come in white or metallic (aluminum) color, and with the plastic splines needed to hold the screening in place. Because the end of each piece has to be cut off at a 45-degree angle to form mitered joints when the corner locks are used to assemble them, Reynolds also makes an inexpensive handy little miter box that can be used to simplify cutting them off at the proper angle.

To make a complete screen frame, measure the window opening on the outside, then cut the sections so that the new screen will be about 1/4-inch less in width and height. Remove the spline, then use the miter box and a hacksaw to cut pieces to length. Use a file to smooth off any burrs or rough edges, then insert the corner locks in the ends and assemble the completed frame. If the frame is more than three feet long, install one of the special center braces about half-way across.

To permit easy hanging and removal of screens, special loop latches can be installed in the bottom section (two are needed for each frame). Put these in place before installing the solar screening because they fit into the same groove as the spline.

The screen mesh is now installed on existing screens (as described earlier), using the same tools and techniques. Matching hanging brackets (hanging channels) are fastened to the window frames to hold the screens in place.