Understanding Window Ratings

Understanding Window Ratings


Buying replacement windows may seem like a simple enough task, especially with all of the options out there.  Choosing can be overwhelming to say the least.  So what information should you consider?  How can you tell that the window is quality?  Understanding window ratings helps customers decide.


But how do you know which window is right for you?

The National Fenestration Rating Council, better known as the NFRC, manages an independent, uniform system for window ratings.  While there are many factors to evalutate, the NFRC focuses on 4 key areas:

  • U-Factor
  • Solar Heat Gain Coefficient (aka SHGC)
  • Visible Transmittance (aka VT)
  • Air Leakage

That’s not to say that other areas of measurement are not important.  Water Penetration, the Design Pressure Rating, Sound Transmission Class Rating, R5 Volume Purchase Program and Condensation Resistance Rating, are also important.  The truth is that all of these areas should be considered when buying windows.  However, it’s easy to get overwhelmed.  So many numbers complicates the comparison process.  Which is why we also refer to NFRC and a limited 4 factors.


Know the Factors

Before we can look at the window ratings system, you should understand each factor.  Here is an overview of each category as explained by the ENERGY STAR® website.


  • U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25. The lower the U-factor, the better the window insulates.
  • Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits.
  • Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.
  • Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf·m/ft².
  • Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows.


The image below provided by the NFRC helps to explain these categories more simply.

window ratings, NFRC label

Closer Look

Now that you understand what the 4 key areas are, let’s take a closer look.   What do the numbers actually mean?

The NFRC separates the label into Energy Performance Ratings and Additional Performance Ratings.  We are all familiar with ENERGY STAR®; a program that lets purchasers know that the item they are buying is energy efficient. The U-Factor, Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) ratings are all required to be considered “energy efficient.” Air Leakage and Condensation Resistance ratings are optional and manufacturers are not required to test their products for these.

As you can see, the NFRC’s label is a great tool in determining which replacement window is right for you. Of course, cost and warranty play big roles as well.