Understanding Window R-Values


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.

Frequently Asked Questions

What is the key difference between R-value and U-value?

R-value measures a material’s resistance to heat flow (higher R-values mean better insulation), while U-value measures the rate of heat transfer (lower U-values indicate better energy efficiency). Essentially, R-value focuses on insulation, whereas U-value considers overall thermal performance.

The National Fenestration Rating Council (NFRC) rates window energy efficiency based on U-value, as it considers the entire window assembly (glass, frame, and spacers). R-value, on the other hand, is typically used for individual materials like insulation or glass panes, rather than full window systems.

Look for windows with:

  • Low U-value (better overall insulation)
  • High R-value for glass (better heat resistance)
  • ENERGY STAR® certification (meets efficiency standards)
  • Low-emissivity (Low-E) coatings (reduces heat loss)
  • Multi-pane glass with gas fills (improves insulation)

No. While R-value and U-value are crucial, other factors also affect efficiency, including:

  • Solar Heat Gain Coefficient (SHGC) – Determines how much solar heat a window allows inside.
  • Visible Transmittance (VT) – Measures how much natural light passes through.
  • Air Leakage (AL) – Indicates how much air escapes around the window frame.
  • Glazing & coatings – Low-E coatings and gas fills (argon or krypton) improve insulation.
  • Single-pane glass: R-value ≈ 0.9 (poor insulation)
  • Double-pane glass: R-value ≈ 1.7 – 3.0 (better insulation)
  • Triple-pane glass: R-value ≈ 4.0 – 6.0 (highest insulation)
    Triple-pane windows typically have a lower U-value and higher R-value, making them more energy-efficient but also more expensive.

Yes! Use the formula:
U-value = 1 / R-value
For example, if a window has an R-value of 5, the U-value is 1/5 = 0.2.

Not necessarily. While a higher R-value improves insulation, it only measures one component (e.g., glass). A window with a low U-value is a better indicator of overall energy efficiency, since it accounts for the entire window 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.