How Window Glass Upgrades Save Energy and Money

Windows are responsible for 25 to 30 percent of residential heating and cooling energy use, according to the U.S. Department of Energy. In the DMV area with hot summers and cold winters, this adds up to significant annual costs.

Energy loss through windows occurs through three mechanisms: conduction through the glass, radiation (heat transfer through the glass), and air leakage around the frame and sash.

Upgrading window glass addresses all three mechanisms: insulated glass reduces conduction, low-E coatings reduce radiation, and proper sealing eliminates air leakage.

Low-E (low emissivity) glass has a microscopically thin metallic coating that reflects infrared heat while allowing visible light through. In winter, it reflects indoor heat back inside. In summer, it reflects outdoor heat away.

Low-E coatings come in different types optimized for different climates. In the DMV area, a balanced low-E coating that blocks summer heat while retaining winter warmth provides the best year-round performance.

Adding low-E glass during an IGU replacement is a cost-effective upgrade. The incremental cost is modest, but the energy savings compound over the 15 to 25 year life of the glass.

Insulated glass units are filled with argon gas between the panes. Argon is denser than air and transfers less heat, improving the insulation value of the window.

Argon gas fill increases the insulation value of a double-pane window by about 15 percent compared to air-filled. It is colorless, odorless, and non-toxic.

Over time, some argon can slowly leak from the sealed unit, but quality IGUs retain most of their argon for 15 to 20 years or more.

The federal government offers tax credits for qualifying energy-efficient window upgrades. Current programs provide credits for windows that meet specific Energy Star performance criteria.

Some utility companies in the DMV area offer rebates for energy-efficient home improvements including windows. Check with your utility provider for current programs.

When planning window upgrades, factor in available incentives. They can significantly reduce the net cost of upgrading to high-performance glass.