R-value and U-value (also called U-factor) are measurements of thermal performance — used to rate how effectively a building element insulates a home. These metrics are important for understanding the energy efficiency of a window unit as well as its compliance with local building codes.
In this guide, our Ring’s End window pros cover the differences between R-value and U-value, what you need to know about each measurement, and how you can find windows with the right performance ratings for your project.
R-Value vs. U-Value
R-value measures the thermal resistance of a specific material (a higher number is better), while the U-value measures the rate at which heat is transferred by a system, such as an entire window or door (a lower number is better).
| R-value vs. U-value | |||
| USED FOR | MEASURES | RANGE | |
| R-VALUE | Building materials | Thermal resistance | 1 - 5 (or higher) |
| U-VALUE | Building systems | Heat transfer | 1.2 - 0.2 (or lower) |
The U-value is displayed on the energy performance label of NFRC-certified windows, and the R-value is usually determined based on the U-value. The measurements are reciprocal, meaning windows and doors with a high R-value will have a low U-value. This is because R-value measures how well the material resists heat, while U-value measures how well the material transfers heat.
Reading an Energy Performance Label
The energy performance label on NFRC-certified windows — such as Andersen windows — displays the U-Factor alongside other key performance ratings.
Solar Heat Gain Coefficient: measures how much heat from the sun is allowed through a window. A lower U-value will mean a lower SHGC as well. Range = 0-1
Visible Transmittance: measures how much natural light is let in. A higher number indicates better performance. Range = 0–1
Air Leakage: measures how much air enters the home through the window, causing drafts. Range ≤ 0.3
What is U-Value (U-Factor)?
A window’s U-value, or U-factor, is the heat transfer coefficient — measuring thermal transmittance. The lower the U-value, the better the window insulates a home from temperature changes and prevents heat loss. A typical U-value range for windows is 1.2 to 0.2. For example, to be ENERGY STAR certified in the Northern climate zone, windows must have a U-factor of at least 0.22.
How to Calculate U-value
U-value is measured in BTU/hr × ft² × °F — a unit of heat transfer rate, describing how much heat (in British Thermal Units) is transferred through a square foot of material for every degree Fahrenheit of temperature difference, over one hour. Or in the metric system, W/m²K (watts per square meter Kelvin).
While the R-value focuses on resistance to conduction (heat moving through solid materials), U-value measures overall heat transfer through conduction, convection, and radiation combined — giving you an idea of how well the window performs as a whole system. Glass option and glass size greatly impact the U-value because the surface area of the glass has the most impact on the formula.
Need Energy-Efficient Windows for Your Climate Zone?
Andersen offers ENERGY STAR certified windows and doors for every climate zone in the U.S. Ring’s End is proud to be an authorized Andersen dealer, servicing homeowners and contractors across the Northeast.
What is R-Value?
“R-value” stands for “resistance” value. This measurement rates a material’s ability to prevent heat flow, given the thickness of the material and a particular temperature difference. (Note that this is different from K-value, also called lambda value, which measures a material’s thermal conductivity independent of its thickness).
How to Calculate R-value
The R-value of individual components can be added together to find a total R-value, but the R-value is typically derived from the U-value when it comes to systems such as window and door units. R-value and U-value are mathematical reciprocals (R-value = 1/U-value), so R-value is understood as the inverse of heat transfer per square foot, per hour, per degree Fahrenheit (ft² × °F × hr/BTU, or m²K/W).
A typical R-value range for windows is 1-5, with high R-value ratings over 5 indicating excellent insulating performance. Unlike the U-value, this number isn’t displayed on window labels.
One of the major differences between R-value and U-value is that R-value measures specific types of insulation materials or insulation products, while U-value measures the cumulative value of systems — such as insulated glass units (sealed panes of glass separated by air or gas).
The R-values of window and door units are usually derived from the U-values, since these systems are made up of so many different types of materials.
How to Improve the U-Factor of Windows and Doors
Andersen A-Series Awning Windows
Configuring windows and doors with the right combination of energy performance features requires experienced planning. For example, homeowners in a northern climate would need windows with a low U-factor to keep the home well-insulated over the winter. But they may also want their sun-facing windows to bring in heat and help reduce energy consumption.
Ordering windows and doors with additional glass panes or glass coatings is an impactful way to improve energy savings and enhance the overall energy efficiency of the home within its climate. Here are the need-to-know details about these glass customization options.
Low-E Glass Coatings
Low-E (low-emissivity) glass coatings are thin layers of metal or metallic oxide applied to the surface of window glass to improve its energy efficiency by controlling heat transfer. “Soft” coatings are applied inside the sealed glass panes of an insulated glass unit (IGU), while a “hard” coating is applied to the outermost surface of the interior glass pane.
Find the Perfect Glass Coating from Andersen Windows
Ring’s End is a trusted supplier of Andersen windows in the Northeast.
Premier window manufacturer Andersen offers five types of glass coatings to improve thermal performance and meet the energy needs of homes in every climate zone across the U.S.:
- Low-E/Low-E4: reflects heat indoors during winter and blocks solar heat in summer
- SmartSun: blocks more ultraviolet rays and further reduces solar heat gain while maintaining visibility
- Sun Glass: features a stronger tint than SmartSun, blocking heat from intense sunlight
- PassiveSun: captures and retains solar heat
- HeatLock: reflects heat back into the room (this is a hard coating)
Dual or Triple-Pane Glass
The number of panes in a window or door clearly have a significant effect on the flow of heat — with single-pane glass providing the least thermal insulation and triple-pane glass offering the best.
The spaces between the glass panes are typically filled with inert gasses like argon or krypton since these gasses are better insulators than air.
By adding layers of glass and gas fills, the rate of heat transfer is reduced because heat has to pass through more barriers. This results in a decrease in U-value — meaning the window is better at preventing heat from escaping or entering.
Since U-value is the reciprocal of the R-value, dual or triple-pane glass also increases the window’s thermal resistance. More layers and gas fills mean it’s harder for heat to pass through, leading to a higher R-value.
Homeowners will notice a significant difference upgrading from single-pane to dual-pane glass, which provides effective insulation in all types of climates. For homes in a Northern climate zone, triple-pane glass can make it simpler to meet strict code requirements.
Pro Tip: The A-Series by Andersen Windows offers a wide range of energy-efficient windows for homes in cold climates. Every window style in the A-Series can be ordered with triple-pane glass.
Find Energy-Efficient Windows at Ring’s End
Most window and door decisions focus on window size, color, and look, but R-value and U-value can be important ratings to understand when choosing energy-efficient windows, particularly in climates with strict building codes. As a window supplier and installer in the Northeast, the window team at Ring’s End knows the importance of installing windows that look beautiful and perform well in harsh winters and hot summers.
Andersen Windows and Doors offers our customers a wide range of quality options that accommodate all types of project needs, and we’re proud to be an authorized Andersen dealer. Visit one of our eight showrooms to experience Andersen windows for yourself and talk with our window specialists in person.