Improving Window Performance

Boost window performance without the high expense of complete replacement.

Windows play a unique role in our houses. They provide views to the outdoors, introduce natural light, deliver passive solar heat, and provide natural ventilation.

But they are expensive. A top-quality window can cost $50 per square foot or more—$600 for an average-size (3- by 4-foot) window. A state-of-the-art, triple-glazed replacement from Germany can cost more than $1,000. Count up the old windows in your home, and you’ll quickly see that replacement can be expensive, often costing thousands—or tens of thousands—of dollars.

With 130 million houses, mobile homes, and apartments in the United States—and a majority of them with poorly performing windows—it’s unrealistic to think about replacing most of them. This article looks at alternatives to replacing windows while still significantly boosting their energy performance.

There are affordable options for reducing heat loss in the winter and reducing unwanted heat gain in the summer and in warmer climates year-round. Some treatments can do both. And if implemented well, such window treatments or attachments can enhance a home’s appearance.

Applied Window Films

Transparent films can be applied to windows to reduce unwanted solar heat gain and reduce heat loss. These plastic films, which are typically made of polyester, are usually applied directly to the interior glass surface, though some products are for the exterior. They are most commonly installed in southern climates to reduce solar heat gain, though some newer, low-e films are more transparent and help reduce heat loss.

While there are window-film kits for do-it-yourselfers available at home centers, the best installations are done professionally by trained installers. Most films are 2 to 7 mils (0.002 to 0.007 inches) thick and come in 36- to 72-inch-wide rolls.

Applied window films are fairly permanent modifications. They cannot be adjusted like most other window treatments, and they are difficult to remove.

The energy performance of applied films is certified by the National Fenestration Rating Council (NFRC). In fact, this is the only window treatment with NFRC certification. Ratings are provided for U-factor (a measure of the heat loss through windows); solar heat gain coefficient (a measure of how much solar heat enters through the window—which can be desirable or undesirable); and visible transmittance (a measure of how significantly films affect visibility through the windows). Tinted films, for example, will dramatically reduce solar heat gain and visible transmittance, but have little impact on U-factor, while newer low-e films may reduce the U-factor (lower U-factor means less heat loss) with minimal impact on the visible transmittance.

Benefits

• For cooling-dominated climates, films are useful for reducing solar heat gain
• Use of low-e films reduces heat loss
• Some films are designed specifically to reduce glare
• Blocks 95% to 99.9% of UV light 
• Enhances privacy (especially films with high reflectance or “mirroring”)
• Enhances security and safety (some films designed specifically for these benefits)
• No operation or maintenance required