Understanding R-Value and U-Value for Replacement Windows

Published January 3, 2021
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Lake Washington Windows

Your windows are part of your interior and exterior. That means that there is a big opportunity for heat and air to flow through them. The more heat and air that flow through this thin barrier between indoor and outdoor the more energy you waste, and the more reactive to the weather your home atmosphere is. To keep your home as comfortable as possible and running efficiently, you will want to consider energy efficient replacement windows need to be.

Energy Efficiency Metric Comparison: U-Value vs R-Value

There are two metrics in the window industry used to measure heat flow and efficiency: 

  1. U-value
  2. R-value

R-value and U-value are the inverse of each other. 

  • The R-value is a measure of heat resistance, and is an assessment of material effectiveness. 
  • The U-value measures heat transfer (loss or gain) through the actual glass. U-value does not rate the specific material like the R-value does. Rather, U-value is a calculation of the conduction properties of various materials that make up the window.

For your replacement windows, you’ll want a high R-value and a low U-value. This would indicate that insulation is working and there are no major air leaks. You want windows that block the sun’s heat and UV rays during the summer while heat stays inside during the winter.

Property U-Value R-Value
Indicates Energy Efficiency Yes Yes
Measure of Insulation Yes Yes
Based on Specific Material No Yes
Based on Multiple Factors Yes No
Indicates Heat Transfer Yes No
Indicates Heat Resistance No Yes
Measures Airflow Yes No
Measures Reflected Heat Yes No
Lower Number is Better Yes No
Higher Number is Better No Yes
On NFRC Label Yes No

What Are Good R-Values & U-Values?

A good R-value range: Greater than R-5.
A good U-value range: 0.20 – 1.20 according to energy.gov.


R-value measures the performance of a specific material, like insulation, to reduce the transfer of heat. We know that the R-value measures heat resistance, but that implies more than is obvious at first glance. The measure of heat resistance of the glass is implies a measure of

  1. Glass energy efficiency
  2. Glass insulating ability

Why R-Value Matters in Windows
The R-value for windows will educate users on what to expect from their performance with blocking air flow in or out.
The R-value will depend on:

  1. The number of panes of glass
  2. The type of frame
  3. The size of the window

Here’s a representation of what R-values could look like:

Type of Pane Winter Summer
Single Pane .9 .96
Double Pane with .25” air space 1.72 1.64
Double Pane with .5” air space 2.04 1.78
Triple Pane with .25” air space 2.56 2.27
Triple Pane with .5 air space 3.22 2.56

As you can see, the more panes and space between those panes, the higher the R-value. R-value can be even higher with the use of Low-E glass, which has a coated finish that reflects heat while allowing natural light into spaces.

Replacement windows with a higher R-value are a great choice to eliminate air infiltration. They have huge benefits over those drafty, cracking ones you may have now. Consider replacement windows not so much as an expense but as an investment in your property. It’s an investment that pays dividends in upgraded curb appeal and energy efficiency.



The U-value is a calculation of the conduction properties of various materials in the window. 

They specify energy efficiency and measure the insulating ability of the window. They are also an indicator of heat transfer. It’s a measure of airflow and radiated or reflected heat. The U-value is used by the National Fenestration Rating Council (NFRC), which certifies energy-efficient and provides information to consumers about the performance of windows, doors, and skylights.

Why the U-Value Matters in Your Replacement Windows

The number provided by a U-value measures the rate at which heat flows through one square foot of material. The lower the number the less heat transfer. U-values are especially important during the winter to ensure cold air doesn’t get in and heat doesn’t get out.

At Lake Washington Windows & Doors, we offer windows in four substrates—clad-wood, vinyl, aluminum, and fiberglass – each of which has its own R and U-value.

Explore all our window offerings, and then get in touch with any questions about optimal R- and U-values for your replacement windows.

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Written by Lake Washington Windows


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