Historically, the term U-factor, which measures the rate of heat transfer of a material, has been used to explain window performance. However questions have risen as to whether the term R-value, which measures the thermal resistance of a material, could be used as an alternative.

Dr. Brandon Tinianov, P.E., LEED AP, chief technology officer with Serious Windows, has written a paper titled “The Use of R-value Versus U-value to Describe Window Performance,” and states “if the use of U-value is established, why would one have a desire to use R-value as an equivalent alternative?”

Tinianov explains that first, “U-values [U-factor] are small, usually less than one, with diminishingly smaller values as performance improves. In contrast, R-values are presented in a number range of highest comfort for a consumer-between 1 and 10 (possibly 20) … Second, the inverse relation of U-value and performance is counter intuitive. As u-value diminishes, performance increases.”

He also states, “There is good public reason and good technical precedence for the interchangeable use of R-value and u-factor to describe windows. With only slight modification to terminology associated with R-value, the public will be empowered to make smarter, more intuitive energy efficiency decisions. Additionally, product manufacturers and building designers will have another useful method of describing their materials in a fair and transparent marketplace.”

Recently, the National Fenestration Rating Council (NFRC) sent out a bulletin explaining why it chooses to use U-factors for windows. In the bulletin, Jim Benney, NFRC chief executive officer, says “From a technical perspective, there are numerous philosophies about whether R-value applies only to homogeneous materials and should be measured in terms of surface to surface heat transfer – i.e., making it the true inverse of conductance – by a guarded hot plate (ASTM C177). Or, should it be used for composite materials and measured in a calibrated hot box in accordance with ASTM C236? Or, should it be measured by means of a heat flow meter (ASTM C 518)?”

Benney continues, “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.”

Benney adds, “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, curtainwalls and fenestration attachment products. NFRC will continue to recognize U-factor-and U-factor only-for fenestration products.”

Some window and energy experts in the industry also have thoughts on the matter.

“U-factor takes into account not just conduction but also airflow, absorption and radiation (emissivity). Unlike most building materials that use an R-value rating and are made up of a single material component (such as insulation, roofing materials, etc.), windows are made up of many components that create the window assembly and the U-factor more accurately measures the heat transfer of this assembly of components,” says Kerry Haglund, a senior research fellow with the Center for Sustainable Research, University of Minnesota. “Because windows allow for the visible and physical connection between interior and exterior, airflow, absorption and radiation are important not only for energy-efficient reasons, but also for human factor issues such as thermal comfort and views to the outside.”

Tom Culp, with Birch Point Consulting LLC, adds, “While R-value does have greater meaning to consumers, there are also technical issues when applied to windows which have the potential to also mislead consumers if oversimplified information is given.”


1 Comment

  1. The comments from the Serious Materials spokesperson demonstrates the fact that they are a marketing company that just happens to make fenstration products. They spend a great deal of effort and money touting R5 and R10 windows that no one is willing to afford. Funny ho wthey never mention how many of these Highly Insulating windows they actually sell. Their dealers claim that the are the same type of outfit as Window World – bait and switch.

    – Guy

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