There’s a lot to think about when choosing replacement windows for your home. You want a style that suits your taste while providing the function you need. You also want a window that will last, reduce energy waste, and offer the best value possible.

That’s where window ratings come in.

All new windows are rated by the National Fenestration Council (NFRC), and these ratings qualify a product from the Energy Star Program. Window ratings also provide the information you need regarding thermal efficiency, durability, sound transmission, and more—so it’s good to know how to interpret and compare these ratings.

Here’s how it’s done.

Comparing R-Value and U-Factor

R-Value and U-Factor measure the thermal performance of the window glass; however, they do not measure equally. R-Value measures the center of the glass only. U-Factor measures the entire glass along with the window frame.

U-Factor marks a tightening of industry standards that began in 2008, but some products hang onto traditional R-Value ratings—just keep in mind that comparing R-Value to U-Factor does not give you an accurate quality comparison.

What is considered a good R-Value or U-Factor rating? For U-Factor, lower numbers are best with a range between 0.17 and 0.39 being optimal. For R-Value, higher numbers are better. Look for a rating between 2.5 and 6.

Interpreting VT Ratings

If you’re concerned about natural lighting, then VT Ratings are important for you. This rating measures visible transmittance, or in other terms, how much light can come in through the window. Though this information is useful, interpreting VT ratings can be tricky when you compare windows with differing features.

Why is that? VT ratings are affected by the design of the window. Frame thickness, window grids, and glass coatings are variables that change the score. So, if you’re shopping for windows with VT Rating in mind, make sure you are comparing the score of windows with like features.

What is considered a good VT Rating? VT Ratings range between 0 and 1, and the higher the score the more natural light you will get from the window.



Sizing Up DP Ratings

Window strength is measured with the Design Pressure (DP) Rating. This rating will protect you from a common sales trick that promises improved durability based on frame thickness alone.

Despite what might seem like a reasonable claim, thicker window frames are not necessarily stronger. It really depends on the material used and the quality of the manufacturing process. So, for an accurate picture of strength, settle for nothing less than the DP Rating from the NFRC.

What is considered a good DP Rating? If you live in a storm-prone area, then you want the highest DP rated window you can afford. A rating between 15 and 50 is considered a quality range.

Understanding STC Ratings

STC ratings deal with sound transmission. As with the DP Rating, STC Ratings protect you from common misconceptions. For example, you might assume that a window with triple-pane glass has better sound proofing capabilities when compared to a double pane window. That’s not always so. Factors such as glass thickness and gas fill have more to do with sound transmission.

What is considered a good STC Rating? Standard windows score between 18-20. Windows that are created for improved sound proofing range between 28 to 48. As you might have reasoned, higher numbers mean better sound proofing.



More Ratings You Should Know About

When you want a window that offers all-around efficiency, durability, natural light, and low sound transmittance, the above scores are a great place to start. Here are additional ratings you might want to keep in mind:


  • CR Rating measures condensation resistance, which is an important efficiency feature. Ratings between 30 and 80 are considered quality performance.
  • Air Infiltration is an important rating for areas that experience strong winds. Low ratings are best; look for 0.30 cfm/sqft or lower.


Do you need help choosing the best replacement windows?

A lot goes into choosing the right replacement windows for your home. If you have questions about these ratings or if you want a window installation cost estimate, contact the trusted window contractors at Mad City Windows today!



No Interest & No Payments Until 2026!

4.9 out of 5 based on 945 Reviews

Get Free Estimate


No Interest & No Payments Until 2026!