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Understanding Your Windows | Beginners Guide to Window Values

December 11, 2018

So you want to buy new windows, but what exactly should you be looking for? Keeping your house temperature well regulated and energy efficient largely depends on quality windows. There are certain values which are essential for you to understand when in the market for new windows. These values are known as the R-value and U-value. We’re here to help you understand these values and get the best replacement windows.

What is R-value?

There seems to be some confusion about these two values, so allow us to break it down for you. R-value tells us how well a particular construction material insulates. The higher the R-value, the better the insulation and the more energy you will save.  An R-value only applies to specific materials, not to systems. A higher R-value means that the window will be more energy efficient when it comes to insulating your home.

What is U-value?

U-value (also commonly called U-factor) is generally used to rate door or window units. The lower the U-value, the more energy efficient the system in question will be. A U-value is typically a low number because it is a rating of how much heat energy is lost or gained. This transfer of energy is measured through both conduction and radiation. A window with a low U-value is more energy efficient, and vice versa.

Window sticker showing example of R-value and U-value

Based on this image, we are able to determine that the window has a U-value of .25. Using the reciprocal property of U and R-values as explained earlier, the R-value of this window example is 4. Since R=1/U, R=1/.25.

The Relation between R & U values

It is important to note that these two values are not directly opposite, but rather they are reciprocals of each other. The equations are as follow U=1/R and R=1/U. Of course, it is not as simple as it seems on the surface. As stated earlier, R-value applies to specific construction materials. This means that an overall R-value is easily obtained by adding the individual components. For example: When determining the R-value of a wall cavity (the area between framing members), you can add the wall sheathing, the insulation, and the internal drywall to get the overall R-value. U-value works quite a bit differently. In a window system, there are different materials with different functionality. Some materials are there to allow airflow, while others are there to stop any heat transfer. Because of this, you cannot just add the values together to obtain an overall U-value.

Why does it matter?

Choosing new windows isn’t only about having better appeal, but functionality as well. Purchasing new windows is a major investment for most people. Getting the most for your money is why these values are important to understanding. Having windows with the best values seemingly guarantee a solid return on your investment; keeping more money in your pocket. They will also help ensure your house stays cozy & warm during the Winter, and cool during the rough Summer heat.


Of course, there are other values and components of purchasing windows that are worth noting. Stay tuned for our upcoming blog post, discussing Solar Heat Gain Coefficient and Low-E glass.


Windows of Wisconsin | (920) 429-9119 |
2300 Tower Dr. | Kaukauna, WI 54130