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.