Why NFRC uses U-factors for windows

Recently, I’ve seen a number of articles, blogs and online discussions regarding U-factor and R-value, especially with regard to fenestration products. It’s time for NFRC to set the record straight on this important issue.

NFRC recognizes only U-factors for energy ratings for important technical reasons, consumer reasons and legal reasons.

This is not a simple issue. From a technical perspective, U-factor is not a material property value. It is the result of a calculation that combines the conductance values of the numerous materials in a fenestration product. This includes glazing materials, gas fills, spacer materials, framing materials, weather strips, sealants, etc. In addition, it includes the convection and radiation elements that occur within and adjacent to the fenestration product surfaces that dramatically influence its energy rating. In thermal chambers, NFRC tests products at specific environmental conditions with tightly calibrated equipment, and also applies a standardized air film coefficient to assure repetitive results from lab to lab.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers has long recognized U-factor as the correct measurement for both fenestration products, and wall and roof assemblies. Only specific materials have a recognized R-value. In addition, the International Energy Conservation Code only recognizes U-factor for fenestration products. ASHRAE 90.1, for commercial buildings, and the IECC, for residential buildings, both reference NFRC’s procedure for determining the U-factor of fenestration products (NFRC 100).

As a 501(c)(3) public service organization, NFRC has an inherent responsibility to communicate to consumers, government bodies and others the most appropriate and credible information about fenestration product performance. Because U-factor provides more technically sound information for fenestration products, NFRC provides U-factors rather than R-values. U-factor is directly related to energy savings because it directly predicts reduced heat transfer. In contrast, the relationship of R-value to energy savings is more complicated and highly variable.

With the energy performance of products assuming increasing importance in today’s marketplace, fenestration product manufacturers face expanded legal risks if they advertise the energy performance of their products in an inaccurate or misleading manner. In fact, the Federal Trade Commission (FTC) has adopted regulations intended to protect consumers from misleading and deceptive advertising practices regarding R-values and home insulation products (70 Fed Reg. at 31,259). However, those regulations give no direct guidance regarding the use of R-values for fenestration products.

It is critically important that product performance is communicated consistently to all interested parties. U-factor is the recognized term for relating the thermal transmittance of windows, doors, skylights, curtain walls and fenestration attachment products. NFRC will continue to recognize U-factor – and U-factor only – for fenestration products.

--Jim Benney is the National Fenestration Rating Council’s chief executive officer. He has been involved in developing product and performance standards for the window and glass industry for more than 20 years. He can be reached at jbenney@nfrc.org.

The opinions expressed here are those of the individual author and do not necessarily reflect those of the National Glass Association, Glass Magazine editors, or other glassblog contributors.


Having represented the legal interests of window/door manufacturers for over 17 years, I can attest to the importance of statements pertaining to the energy-saving properties of fenestration products. It is a matter demanding a thoughtful approach. NFRC represents a very significant resource for understanding and dealing with the issues.Paul Gary
Thank you all for your comments. It is your input that makes glassblog a useful forum in which we can discuss important issues. When posting comments, please refrain from name-calling, profanity and other offensive language. Thank you, and please keep the comments coming!
The problem I have with U vs R is the consumer over the years has come to know R value as a meaningful way to determine how well something is insulated. The higher the R, the better the situation. Talk U value with a customer and they glaze over like you are speaking a foreign language. They want to know what the R value is. Honestly, I think if they knew the approximate R value for most windows, they'd put less windows into their projects.
I agree completely with the direction the NFRC is taking in this important issue. Having watched the window industry create problems for itself with outrageous claims of energy efficiency, this is exactly what is needed. The point of diminishing returns when the R is stated as going from 1 to 2 and then 5 to 10 is a perfect example of how misleading this can become. No wonder, home improvement contractors are sometimes considered unethical or "Tin Men" when numbers like these can be so skewed to serve whatever purpose. Taking this stand in the long run will benefit all of us.
Thanks for the article and explanation. This is exactly what our industry needs, education and exposure. The reason people relate to the R value is because it has been marketed and taught, now we need to do the same for the U value.Tracy Shaver
NFRC conducted focus groups earlier this year with consumers, interior designers and architects. The industry definitely needs to educate consumers about u-factor - only a few knew what it meant - but all the designers and architects were very familiar with the term and what it measures.
Leonard, Just be aware that "consumer expectations" have to be managed so as to be realistic.Paul Gary
Excellent explanation of the differenece between U and R and why we should all be using the same criteria. Meanwhile agencies outside our industry are trumpeting the value of R-5 windows and encouraging us all to get on board.
I respectfully disagree with the commenter’s conclusion that consumers would purchase fewer windows. Homeowners want (and need) windows, doors and skylights for many reasons, including daylighting, a view, a connection to the out-of-doors, security, ventilation, egress, productivity and a better sense of well-being. Not to mention the warmth of the sun coming through the window on a cold winter's day. How do you rate all of these benefits? R value should be used to compare insulation; but don't compare walls to windows. NFRC certified U-factors provide a way to compare windows to windows, and to make sure that they meet required energy codes.