When researching replacement windows, you’ve likely stumbled across two terms relating to energy efficiency: R-value and U-value. Both terms are used in the construction industry to define the energy efficiency of a given material, but there are key differences that can be confusing to understand. Choosing the most efficient windows starts with understanding the differences between R-value and U-value, and their relationship to a window’s overall energy efficiency.

What Is R-Value?

R-value is commonly used to define the energy efficiency of a building material. In fact, the term was originally developed for measuring the thermal performance of insulation. When the R-value of insulated glass is higher, it performs better. Materials with higher R-values provide better insulation, or more technically, increased resistance to heat flow. 

Thus, a material with an R-value of R5 is more energy efficient than an R1 material. R-value is also the inverse of U-value. For instance, you can determine a window’s U-value with this equation: 1/R-value = U-value. In other words, more energy efficient windows have lower U-values.

In terms of assemblies, R-value of windows is less commonly used, because it refers to a specific material component of a window, i.e. solely the glass pane. On the other hand, U-value is a measure of the insulating value of an entire assembly, including the frame, hardware, spacers, glazing and glass. In fact, the National Fenestration Rating Council (NFRC) – which rates the energy efficiency of windows – includes just U-value in their ratings that appear on window labeling.

More recently, window manufacturers have started developing energy efficient windows with R-value ratings. For instance, manufacturers now produce assemblies labeled R5 windows, which are extremely efficient and have a U-value of .2. One reason for the term being used is that it’s easier for consumers to grasp, i.e. an R6 window is better than an R5 window, compared to a 0.15 window trumping a .2. Today, though, U-value remains the industry standard for energy efficiency, and generally, it is much more commonly used to define energy efficiency.

How Does R-value Relate to Window Assemblies?

R-value of glass can be used to define the efficiency of a specific component. For instance, a window’s frame would have a specific R-value, while the glass, or glazing, would have a different value. Here’s an example: Single-pane R-value of glass that is one-fourth inch thick is around .90, while R-value of a double-paned window of similar thickness would be nearly double at roughly 1.7. Thus, the window’s R-value is referring to that one specific component.

Because R-value is a measure of a specific component, U-value is the better unit for comparison; a low U-value entire window assembly is more energy efficient than one with a higher U-value. On the other hand, the R-value of window glass can only be used to compare the glass, not the entire assembly. In some cases, though, the term “whole window” or “full-frame” R-value of glass may be used to define the measurement of the entire assembly. In fact, the R-5 window rating, as mentioned, is a measure of the entire assembly.