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Understanding R-value and U-value for Doors and Windows

Thursday June 27th, 2019

What is R-value? What is U-value?

Window and door R-values are common questions that arise when purchasing replacement windows or adding a new barn door or front door to your home that has glass in it. There are two basic terms that refer to the energy efficiency of a window or door; R-value and U-value. Let's begin with the R-value and U-value of windows.

Both of these terms, R-value and U-value, are commonplace terms in the remodeling and construction industry and are used to define how energy efficient a material is. R-value is used to define the energy efficiency of a particular building material and originally was developed for measuring the thermal performance of insulation. The United States government created regulations around energy efficient building materials and mandated that values be added to materials for more efficient use of natural resources. Materials with higher R-values provide better insulation and are able to resist heat more than something with a low R-Value. Materials that are very inefficient are listed out with low R-values such as R-1, where more energy efficient materials are listed out at R-5. Keep in mind that this is the opposite for U-values. U-values are used to determine the insulating efficiency of the window and all of its components. This is different than R-value that would be used to determine the energy efficiency of just the window pane, thus a window with a U-value of 1 is more energy efficient than an window with a U-value of 5.

What is the National Fenestration Rating Council?

Often referred to as the NFRC, the National Fenestration Rating Council actually determines the energy efficiency of windows. When determining the energy efficiency for windows the NFRC doesn't even list an R-Value, rather only listing the U-value rating on the window label. When referring to architecture, the word Fenestration is defined as the arrangement of windows and doors on the elevations of a building.

Parts of a window, or door with a window installed inside of the door, which are assessed for the U-value rating include:

-Window or Door Frame
-Window or Door Hardware
-Window or Door Spacers
-Window Glazing and Glass

What is the best R-value for windows and doors?

Improved education and awareness of the cost of heating and cooling that escapes from poorly insulated components and glass panes have led to window and door manufacturers developing energy efficient windows and doors typically labeled "R5 U.2" meaning the glass is the most energy efficient possible at a "5" rating and the window or door with all of it's built out components are a ".2" which is almost as low as any rating can go (remember a 5 is the highest energy rating for R-value which is what you want, and a .1 would be the very lowest for a U-value rating since U-values are listed as the lower the number the better). To make things more simple for consumers, R-value and U-value are typically combined

Here is a simple chart to help understand how R-value is calculated for windows and doors:

 Single replacement window glass
 Single replacement window glass for storm applications
 Double insulating glass
   (3/16” air space)
   (1/4” air space)
   (1/2” air space)
   (3/4” air space)
   (1/2” w/ Low-E 0.20)
   (w/ suspended film)
   (w/ 2 suspended films)
   (w/ suspended film and Low-E)
 Triple insulating glass
   (1/4” air spaces)
   (1/2” air spaces)
 Addition for tight fitting drapes or shades, or closed blinds    

 Wood Hollow Core
 Solid Core Flush
 Panel Door w/ 7/16” Panels
 Storm Door
   (wood 50% glass)
 Metal Insulating
   (2” w/ urethane)

Rustica's french style sliding barn doors, entry doors, and Rustica's replacement windows have the most efficient R-value on the market listed at 3.94 for interior doors and replacement windows and 5.31 for a 2-1/4" exterior door.

Look at Rustica's energy efficient sliding french barn doors here
Check out our energy efficient front doors here 

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