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.
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 3.6 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.
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 1.69. Increasing the air space between the panes to 3/4 inch increases the R-value to 2.38. Adding Low-E glazing bumps the R-value to 3.13.
Adding a third pane of glass to the window assembly increases the basic R-value to 2.56 when 1/4 inch spacing is utilized. Doubling the air space to 1/2 inch increases the R-Value to 3.23.
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.
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