Understanding Window R-Values

So, you're in the market for new windows and you suddenly realize – What are all of these ratings and values? Can't I just buy a new window and call it a day?

It's true, shopping for windows these days can be a lesson in patience…and science. One of the most important things you are going to want to look for in a new window is its R-value. This guide will help you get a better understanding of what R-values are and why they are so important.

What is a Window's R-Value?

The R-value (or R-factor) of a window is a rating system used in the building and construction industry that indicates a window's thermal resistance. It is based on a set of standards as outlined by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE). The higher the R-value number, the better the window is at resisting heat. This is similar to, but not to be confused with, the R-value of home insulation.

How is R-Value Determined?

R-values are determined by five primary factors. These include:

  • The number of layers of glass used
  • The amount of air space between the layers of glass
  • The type of glazing material used
  • The tightness of the installation
  • The thermal resistance level of the materials used including the spacers and frame

What Part of the Window does the R-Value Relate To?

The R-value is the heat resistance capabilities determined for the entire window – glass, frame and all.

What is the Difference Between a Window's R-Value and Its U-Value?

Whereas the R-value rates the window's resistance to heat, the U-value rates the window's tendency to let heat enter or exit. A well-insulated window will feature a high R-value and a low U-value. Typical R-values range between 0.9 and 3.0 and U-values are usually between 1.1 and 0.3.

Dave Donovan is a freelance copywriter living in Atco, NJ. An electrician for 15 years, an injury forced him to pursue his true passion - writing.