Low-E glass is best when considering energy-efficient replacement windows, but window film can also improve your home's temperatures.
Direct sunlight might be a good thing for your tomato plants, but it's not necessarily welcome when it's blasting through your home's windows and making your A/C work overtime. Homeowners have a few options to achieve better energy efficiency with windows, including installing window treatments, window film, or low-E glass replacement windows.
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Jim Hardy, president of Clear Choice Windows and More in Largo, Florida., says low-E glass is the way to go. "A lot of times, you'll void the manufacturer's warranty if you put tint on a regular window," he says.
Kevin Koval of Solaris in Carmel, Indiana says you won't lose your window protection if you choose a dealer who offers a window film that replaces the window manufacturer's warranty.
"Every window manufacturer warranty is different. A lot of them will be voided if you put window film on it. Some [window film manufacturers] match the warranty to the letter."
How low-E glass works
The effectiveness of low-E glass is measured in two ways: the U-factor (the window's resistance to heat loss) and solar heat gain coefficient (how well the window blocks heat). Hardy suggests checking documentation carefully to ensure both numbers are 0.30 or less in order to get the most energy efficient window.
"The lower the number, the more heat it blocks," he says.
Kirk Morgan, owner of Morgan Exteriors in Lutz, Florida, also strongly recommends low-E glass, and suggests avoiding double-pane glass with metallic spacers.
"Metal spacers conduct heat and will eventually fail," he says. "You want a silicon foam spacer system. A good window will cost more upfront, but it's less expensive and less hassle over the years to deal with."
Jon Koloms, vice president of Scientific Home Services in Skokie, Illinois, says low-E glass blocks the sun's brightest and heat.
"The U-factor is particularly important in the Midwest because it tells you how well the window keeps heat inside [during winter] and reduces heating costs," he says.
Koloms also recommends looking for glass that blocks ultraviolet rays, which cause furniture, drapes and carpets to fade.
When to consider window film
Window film is a good alternative if you want more energy efficient windows without the replacement costs. Koval says you can use window film if your window's are structurally sound. You can choose from thousands of films to get the result you want. Some window film can block heat, glare or UV rays. Some can achieve all of those goals and help your home retain heat in the winter.
Koval says window film is a good option if you want to make a particular room or area of your home cooler or warmer. Also consider window film if you have large picture windows you don’t want to replace.
"It’s [window film] custom cut to fit your existing windows, so any shape or size can be done at a fraction of the cost to replace a window,” Koval says.
Editor's note: This is an updated version of an article originally posted on June 16, 2010.
Have you installed window film or low-E glass replacement windows? Tell us how it affected your energy bill in the comments section below.