QUESTION: I saw some do-it-yourself window film at my home center store. Some looked almost clear while others were darkly tinted. Will applying these reduce the heat and fading through windows and which type is best?ANSWER: Windows, especially those which face south and west, are one of the greatest sources of heat gain during the summer. This not only makes your family uncomfortable and fades your furniture and carpeting, but it also increases your utility bills by making the air conditioner run more.People normally think of installing permanent window film to block the summer heat, but some types are also effective during the winter. These year-round window films have a low-e (emissivity) coating similar to efficient replacement windows. This also keeps the heat indoors during the winter.When determining which type of window film to buy, it helps to understand a little bit about how it works.When the sun's rays hit the window film, some is reflected away. Some of the light is converted to heat as the film absorbs it. The rest of the light and heat pass through into your home.The darker films that you saw tend to absorb more of the sun's rays. This heats the glass and much of the heat flows back outdoors. This may make the glass get quite warm, so it is not good to use on double-pane windows. The heat may cause the air between the panes to expand and the seal may fail. Check your window warranty about installing film.The films that looked almost clear may or may not be low-emissivity. Some may be just highly reflective. This is not bad, but you may not like the mirrored appearance it gives your windows.From indoors during the day, you can barely notice this film has been installed. The more reflective window films are very effective during summer and the glass does not get excessively warm.The year-round low-e window films will have only a slightly reflective appearance due to the efficient coating, but it is not excessive. You likely will not be able to detect the window film from indoors or outdoors. The low-e coating is actually metal which is applied only several atoms thick. Compare the emissivity specifications on the packaging.These year-round films are good for all windows, especially ones on the north during the winter. They are also particularly effective for windows over a patio, sidewalk or driveway. The low-e coating will help block the infrared heat which is radiated up from the hot concrete or asphalt areas.Do-it-yourself window film is available in rolls or in precut kits. It is not difficult to install yourself, but you may choose professional installation on very large windows. Bottles of liquid adhesive remover are available if you ever want to remove the film.The following companies offer energy-saving window films: Bekaert, (800) 282-9031, www.solargard.com; CP Films, (800) 528-1290, www.cpfilms.com; Film Technologies, (800) 777-1770, www.filmtechnologies.com; Madico, (800) 225-1926, www.madico.com; and Plastic View International; (800) 468-6301, www.pvifilm.com.James Dulley has a degree in mechanical engineering. His Sensible Home column appears in 400 newspapers.