Low-E glass or window film: Which blocks heat better?
Window film installation should be relatively simple and take no longer than a day for a large house. (Photo courtesy of Advanced Film Solutions)
When the summer sun penetrates windows, your home's inside temperature rises — along with your cooling bill. Among your options to keep cool are installing low-E glass or applying a reflective film.
But, which works better?
Angie's List member Roger Linville submitted this question: "My front windows face southwest. I have no shade trees in the front yard, so I'm expecting my front rooms to get hot this summer. I also have condensation between the panes. I plan to replace the windows but am getting conflicting advice. Should I get low-E (low-emissivity) glass and rely on it to block infrared heat waves, or get clear glass and have an infrared-reflective film applied? Which will give me the best results?"
The majority of window professionals we spoke with pushed replacing windows with low-E glass windows. You get the best efficiency by replacing the entire window, because you also replace the frame’s weatherstrip and seal, says John Nelson, owner of window replacement company Austin Retrofit in Austin, Texas.
Most agree that window film blocks heat effectively, but aftermarket film comes with drawbacks.
Energy effecient windows
When looking at energy-efficient windows, look for two key performance indicators: the U-factor and the solar heat gain co-efficient. The U-factor indicates a window's overall insulating value; the solar heat gain co-efficient measures how well a window deflects incoming solar heat. An efficient window should be rated at 0.30 or below in both measurements.
Mike Van Gundy, owner of Empire Window Co. LLC in Delaware, Ohio, advises that the low-E coated glass products that use an argon gas filling in between the glass are the best option.
For hotter climates, Mike Gilkey, owner of Gilkey Window Company Inc. in Cincinnati, suggests installing window glass with three low-emissivity coatings already built in. With LoE3, the triple stack coating goes on the exterior plate of the inside surface of the glass, so it's protected from the elements, which prevents it from tarnishing.
Energy-efficient windows will start at about $300 to $400 and could go up to $1,000. You may qualify for a federal tax credit by installing new, energy-efficient windows. (For more information, visit energystar.gov.)
Although pricey, experts say the investment will pay for itself over time.
Window film effectiveness
Compared to low-E windows, the cost of installing tinted film on clear glass is less expensive overall. David Parke, a sales manager at Custom Sun Control in Marietta, Ga., says most solar reflective films start at about $6 to $14 per square foot. "For the money, you won't find a window that will give you the same performance as solar film," he says. However, installing the reflective film won't address an inefficient window frame.
Heat blocking window film also won't provide the efficiency of the newest triple layer low-E coatings that are sprayed onto the glass at the factory and then cooled to create a bond.
Applying aftermarket films to single-pane glass works to block infrared and ultraviolet light, but it's not a good idea with dual-pane glass because the film is applied to the inside of the window. Because of this, heat energy passing through the dual-pane glass is reflected back inside the insulated glass and can cause excessive heat buildup inside the unit, resulting in condensation.
Also, applying the film to a window may void the glass manufacturing company's warranty, because the film can sometimes cause seal failure in the glass.
Editor's note: This is an updated version of a story originally posted July 12, 2010.