Storm Windows: Clear Glass or Low-E?

Up to 25 percent of your heating bill can be due to your windows, according to the U.S. Department of Energy. Many people think that it is necessary to replace old single-pane windows with new windows to save energy. However, storm windows placed over the window on either the inside or outside will create greater energy efficiency without having to replace the original window. “Adding a storm window to a weather stripped historic window can achieve essentially the same, and sometimes better, energy performance as a new insulated dual-pane window,” advises the U.S. Environmental Protection Agency (EPA).

Image from BuildingGreen.com

Anyone ordering storm windows from Boston Building Resources will need to decide whether they want windows with or without low-e coating. Clear storm widows are simply affixed to the exterior casing of the house outside the primary window to create a pocket of air between the windows, insulating the home. According to the Department of Energy, low-e (short for low-emissivity) windows do the same thing, but a minuscule metal coating reflects heat back into the house while still allowing light to enter. Because of this, low-e storm windows can be more financially beneficial than clear storm windows.

At Boston Building Resources, a clear storm window costs between $161 and $198, depending upon the size, plus installation. Adding a hard-coat low-e coating is an extra $30. In a Chicago study, clear storm windows saved about 13 percent of heating energy, and low-e storm windows saved about 21 percent. This same study concluded that clear storm windows have a simple payback of between 8.4 and 12.1 years, and low-e storm windows have a simple payback of between 3.5 and 5.1 years. This showed that low-e storm windows are more cost-efficient.

Low-e windows do limit the visual quality of the window slightly. When compared to a clear window close up, it is difficult to see a difference. But when the windows are viewed from farther back in sunlight, the low-e coated window has a slight grayish tint. Low-e coating also reduces the amount of visible light that enters the interior space (technically known as visible light transmittance). Harvey Building Products, the manufacturer of Tru-Channel storm windows, uses Solarban® 60 Low-E Glass from PPG Industries, which allows 70 percent of visible light to pass through. Compare this to 80 percent for two panes of clear glass.

Low-e windows work ideally in very hot or cold climates to maximize their insulating quality. According to the Department of Energy, low-e windows come in two types: high solar-heat gain, which maximize solar heat in cold climates; and low solar-heat gain, which keep out solar heat for cooler homes in hot climates. Both types will work well in all seasons because of their insulating properties. A high solar-heat gain low-e window, while meant for cold weather, would still help keep a home cool in the summer.

Sometimes, historical preservation codes prohibit changing the exterior appearance of the window. In this case, interior storm windows can be considered. Exterior storm windows could also interfere with the operation of historic windows. If the window swings out or has an awning, then interior storm windows might be necessary. It is important to consider the specific window before deciding how to proceed.

Storm windows use hard-coat low-e, while most modern windows are made with soft-coat low-e. According to Goldman Remodeling and Phase Developments, soft-coat low-e is thinner, though both are incredibly thin. Soft-coat low-e also prevents more thermal energy from passing through, making it more efficient. Soft-coat low-e also has more optical clarity. While soft-coat low-e is better, it cannot be applied to storm windows because it is harder to manufacture and isn’t as durable.

Low-e storm windows are most effective when there is potential for a lot of energy to be lost. For this reason, low-e storm windows may not make financial sense in a seasonal home where the heat isn’t on most of the season or in an unheated place like a garage or porch. Before purchasing low-e storm windows, it is important to evaluate the amount of energy that they could prevent from being lost.

Low-e storm windows are a good option when trying to improve energy efficiency. They are less expensive than replacement windows and allow homeowners to keep their original windows. Overall, they can improve a home’s energy efficiency at a lower cost than replacement windows and are more financially prudent than clear storm windows.

—Kelly Gallagher