Replacement Window GuideA single-pane window is one layer of glass with no insulating cavity, while a double-pane window seals two layers around a gas-filled space. ENERGY STAR reports that replacing old single-pane windows with certified windows lowers household energy bills by an average of up to 13 percent nationwide. This guide compares the two, with every figure tied to its source.
What separates the two
The NFRC glazing guide describes a double-glazed window as two layers of glass separated by a layer of inert gas that significantly reduces heat transfer.
A single-pane window has none of that, just one layer of glass between inside and outside. The sealed gas cavity is the core difference, and it is what gives a double pane its insulating advantage.
How much single panes lose
The NFRC glazing guide states that homes can lose up to 30 percent of heating and cooling energy through inefficient windows, and that proper glazing can reduce heat transfer by up to 50 percent.
Single-pane windows sit at the inefficient end of that spectrum. With only one layer of glass and no gas fill, they let heat pass far more freely than a sealed insulated unit.
Heat and fading through single glass
ENERGY STAR notes that a standard double-pane window allows approximately 75 percent of the sun's heat into a home, while certified options substantially reduce that heat gain. A bare single pane offers even less control over that solar heat.
ENERGY STAR also reports that the coatings on certified windows reduce fading by up to 75 percent from ultraviolet light exposure, protection an uncoated single pane does not provide. Upgrading adds both heat control and interior protection.
The savings from upgrading
ENERGY STAR reports that replacing old single-pane windows with certified windows lowers household energy bills by an average of up to 13 percent nationwide.
ENERGY STAR also requires that certified windows meet U-Factor and, where applicable, SHGC requirements based on climate zone. Because single panes start from such a low baseline, the switch to a certified double-pane unit is where much of that saving comes from.
A lower-cost middle path
You do not always have to replace a single pane to improve it. The Department of Energy reports that installing a low-e storm window over a low-performing window can reduce a home's heating and cooling costs by 10 to 35 percent, at about a quarter of the cost of full replacement.
That makes a low-e storm window a practical option for keeping an original single-pane window while capturing much of the comfort gain. Full double-pane replacement remains the choice when the existing window is failing.