The Differences Between the R-Value in Single & Double Pane Glass

Heat lost through windows can amount to up to 25 percent of a winter heating bill, according to Michigan State University Extension. Improving the insulation quality, measured as R-value, decreases utility costs and reduces the carbon footprint and emissions of the home. The R-value of the glass is part of the overall energy efficiency rating of the window.

Single Pane

The more panes of glass in the window, the greater its insulating value.

A single pane of glass has an R-value of about 1.  Homeowners often double this by adding a single-pane storm window. These insulation ratings compare to blown-in cellulose insulation with an R-value of 36 per inch of insulation.  In roofs and ceilings, as much as 10 inches of insulation may be blown in to provide an R-value of nearly 40.

Double Pane

The R-value of double-pane windows varies depending on the thickness of the glass and if a low emissivity or Low-E glazing is used.  A simple double-pane window with 1/4-inch spacing is less energy efficient than a single-pane window and storm at an R-value of 169. Increasing the air space between the panes to 3/4 inch increases the R-value to 238.  Adding Low-E glazing bumps the R-value to 313.

Triple Pane

Adding a third pane of glass to the window assembly increases the basic R-value to 256 when 1/4 inch spacing is utilized.  Doubling the air space to 1/2 inch increases the R-Value to 323.

Addition Window Insulation

Insulating films suspended between the glass of double and triple pane windows adds to the overall insulation, increasing the R-value by as much as 1 over the base insulation of the panes.  The most energy efficient windows include both the suspended film and the Low-e glaze, and can reach an R-value above 4 in some circumstances. In addition, the fit of the window and the absence of gaps where cold exterior air can infiltrate into the home is important for energy efficiency. 

About the Author

Keith Allen, a 1979 graduate of Valley City State College, has worked at a variety of jobs including computer operator, medical clinic manager, radio talk show host and potato sorter. For over five years he has worked as a newspaper reporter and historic researcher. His works have appeared in regional newspapers in North Dakota and in "North Dakota Horizons" and "Cowboys and Indians" magazines.