Windows let the sunshine in, but in many cases, they also let the heat in (or out, in the winter). According to the U.S. Department of Energy, heat transfer through windows can account for 10 to 25 percent of your heating and air conditioning costs. Older, single-paned windows are the biggest energy wasters.
Replacing older windows with energy-efficient ones can be expensive, but doing so will save you money in the long run by reducing your energy use as much as 30 percent. Energy Star-rated windows are twice as efficient as typical models sold just 10 years ago. A variety of factors determine a window’s energy efficiency:
Solar heat. A window’s solar heat gain coefficient (SHGC), a value ranging from 0 to 1, indicates the fraction of incoming solar radiation admitted through a window. The higher the number, the more heat will be transmitted, so you want a high value in colder climates (to take advantage of free heat provided by sunlight) and a low value in warmer climates (to keep heat out and lower cooling costs).
Heat transfer. The rate of heat transfer between the inside and outside of a home (unrelated to solar radiation) is known as a window’s U-factor, which generally ranges from 0.2 to 1.2. The lower the value, the less heat is lost from your home — especially helpful during the winter. Some double- or triple-paned windows also contain argon, an inert gas, between the panes to minimize heat transfer.
Glazing. Most energy-efficient windows are coated to help reduce heat transfer. Low-emissivity (or “Low-E”) coatings, composed of microscopic metal particles, reduce heat transfer by 40 to 70 percent while still allowing most light through. Tinted and reflective glass are also available, but they block some incoming sunlight as well.
Framing. Aluminum is a poor choice for window frames because it conducts heat readily.
Windows purchased in 2009 or 2010 that meet specific efficiency criteria are eligible for a federal tax credit equal to 30 percent of the purchase price (up to a maximum of $1,500 for all qualifying home improvements).
How to Make Old Windows More Efficient
If you can’t replace your old windows now, there are other steps you can take:
- Seal air leaks around windows with caulk or weatherstripping.
- Affix Low-E coated film directly to windows to help reduce heat loss.
- Install storm windows to reduce heat loss from single-pane windows by 25 to 50 percent. Low-E storm windows can cost less than a new energy-efficient window.
- Use insulating window treatments including shades, curtains, blinds or awnings to block incoming sunlight in summer and keep heat in during the winter.
The Union of Concerned Scientists is the leading science-based nonprofit organization working for a healthy environment and a safer world. Founded in 1969, UCS is headquartered in Cambridge, Mass., and has offices in Berkeley, Calif., Chicago and Washington, D.C.