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replacement windows in Phoenix, AZ

Understanding the U-Factor on Replacement Windows


If you have been shopping around for replacement windows in Phoenix, AZ you may have heard terms like R-value and U-factor. If these seem particularly perplexing, know you are not the only one who may not understand what they mean. These terms are similar in that they are used to determine efficiency. But understanding the difference is the only way to help determine which products offer the best performance. So here is a bit of information to help you understand these terms a little better.

What is the R-Value?

Originally the R-value was developed to rate thermal performance on insulation, a homogenous material. But today the R-value is used to rate efficiency on a variety of different building materials. A higher R-value means better resistance to heat transfer. So a product with an R-value of ten preforms better than a product with an R-value of six. That said, because R-values are used to measure homogenous materials, R-values on windows often refer to just a specific part of a window instead of the entire assembly. That is why the National Fenestration Rating Council (NFRC) uses the U-value on its energy rating labels.

What is the U-Value or U-Factor?

In basic terms, the U-value is typically the mathematical reciprocal of the R-value. So the formula is U=1/R, and as the R-value increases the U-value decreases. But the U-factor is a little more complex than that. When comparing products, a U-factor of 0.06 has a better energy performance than a U-factor of 0.09. But instead of just a single material, the U-factor measures heat flow per square foot and can measure the rate of heat flow through the entire assembly. So it accounts for how energy enters and leaves the material, the conduction, and the radiation. It also measures airflow and convection around the window.

What if the Window Has Both Ratings?

While the industry standard on energy labels is still the U-factor, many manufactures have started labeling their own products with R-values. This is because it is generally easier to understand how an R9 has better energy performance than an R6, compared to understanding the difference between 0.11 and 0.06. But still, it is important to remember the R-value measures the efficiency of a single component, like the window frame or the glass. While it may be a good piece of information, the best way to compare products will still be the U-factor because it rates the entire assembly. So be sure you check the NFRC energy labels on replacement windows. There you will also see ratings for the Solar Heat Gain Coefficient (SHGC) as well as Air Leakage (AL) and Visible Transmittance (VT).

If you still have questions about R-value and U-value or don’t feel confident navigating the energy labels on replacement windows in Phoenix, AZ, contact Freelite Inc. Our window experts will help you sort out which products are best for the local climate. They will also help you find the right window frame material and window design for your home and budget.