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.

Contact Aeroseal 

For expert advice and leading commercial window and door installation services, contact Aeroseal. Our team can help you select the best technology for your building, ensuring optimal performance and long-term satisfaction. Reach out today to learn more about how we can assist with your window and door needs.