Should We Use R-Values?

John G. Swanson
April 6, 2010
THE TALK... | Codes & Standards

Survey Results as of 04/12/2010:

Should we change to R-values to rate windows and doors?









The industry appears to be fairly evenly split on the issue of whether using R-values in place of U-factors would be a good thing. I can't remember quite such an even split on an issue. And judging by the number of responses I got this week, I would say there are strong opinions on both sides. Some think a move to R-values makes sense, because consumers understand them better, while others assert that U-factors are a more technically accurate way to look at product energy efficiency.

Before looking at some of those opinions, I wanted to thank those respondents who took the time to provide a fairly simple explanation of the differences between the two terms. As I noted last week, R-value and U-factor are reciprocal, “but only when applying them to continuous, frameless, material layers,” wrote Roger LeBrun, product certification engineer for Velux USA. “For a non-homogeneous, unitized item like a fenestration product, there are boundary conditions that change the conversion to something other than a straight reciprocal. Just look at the IECC U-factor equivalence tables, R-value rated materials can be converted to U-factor by reciprocal, but you will not find a table where U-factor rated items are converted to R-value. It is simply technically incorrect, and (probably) misleading, to do so.”

David Cooper, product engineering manager at Guardian Industries wrote to say “the simplest way” to explain the difference between the two measurements are as follows:

“U-factor is the sum of the individual coefficient of heat transfer components in a system and is generally measured with the air film on either side—which adds two additional components. The R-value, which is the resistance to thermal transfer and is equal to 1/U, generally does not include the air film and is a measure from surface to surface. However, one can simply add the air film to the R-value and state it as such—e.g. R-8 with air-film.”

“The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss," writes Andrea White, a regional manager with Sierra Pacific Windows. "R-value is a measure of conductivity. U-factor, takes into account more than conductivity. It also is affected by the airflow around the window and the emissivity of the glass.”   For those looking for further explanation, White suggests a U-factor fact sheet from the National Fenestraiton Rating Council Web site.

In addition to helping with the explanation, White wrote to say she thinks the industry should stick with the use of U-factors. “Architects, builders, contractors and consumers need to understand that fenestration products will never be in the same category as wall insulation etc, and should be thought of differently,” she says. “The entire envelope needs to be adjusted to improve energy performance: percentage of glazing, size of glazing units, where the glazing is placed considering the direction/elevation of the building, use of shading, etc. The best energy efficiency is maximized when of all of these factors are considered and ‘tuned’ to the specific project.”

“Most people think in terms of what terminology they know,” writes Gary Williams, a Texas-based building materials supplier. “So, they ask what is the R-value of a window or door. “He doesn’t necessarily advocate a change but credits Owens Corning with doing a great job educating the public on R-values in the early 70s.  Most agree that homeowners have a better understanding of R-values in general. Thom Zaremba, a glass industry consultant, suggests using R-values would be very beneficial, making it easier to communicate the difference between various glass products.

“From a personal perspective, I like to think in terms of R as it is easier to compare an IG unit that has an R (center of glass with air film) of 11 with one that has an R value of 6,” notes Guardian’s Cooper. “Most folks know their stud wall with (Guardian) insulation is rated at R-13 to R-21 depending upon their zone, or their attic insulation should be R -30 to R-60. And most folks can easily figure out higher is better. This becomes important as the fenestration industry is educating the general public.”

“Since these values are virtually a mathematical inverse, either number is okay for us as ‘professionals,’ however, the use of R-values as a relevant, understandable sales tool (number) to all of our customers is a smart move,” says Tom Kunkel of Bradco Supply, who notes he has been selling windows and doors since 1986. “Everyone can quantitatively understand the relative insulating value if expressed in R terms. Not so with U.”

“I think it makes a whole ton of sense for the entire building envelope to use the same terminology as far as rating insulating value,” says Tim Weisensel of Hometown Glass & Improvement in Wisconsin. Despite some evidence of support for the use of R-values from glass industry representatives, he thinks U-factors are used partially to keep the public unaware of the insulating value of windows and doors compared to other wall and insulation products. “Windows would take a serious PR hit and sales could further decline.”

Others think converting U-factors to R-values for homeowners would be deceptive, however. “A change from U-value to R-value is nothing more than a ploy to overstate and exaggerate marginal changes in window performance,” writes David Paulus of Wasco Windows in Wisconsin.

“The best way to see this is with an example. Consider four windows systems, with NFRC U-factors of 0.4, 0.3, 0.2 and 0.1 BTU/hr-ft^2°F, respectively,” he writes. “For a given size window, moving up to the next more efficient window will save the exact same amount of energy. That is, each move to the next better window saves approximately 2.4 BTU per square feet of window area for each heating degree day. If we were to look at the same windows using R-values, they would have values of 2.5, 3.3, 5 and 10 respectively. It should be clear how this is used to mislead consumers. The U=0.3 window is only ‘0.8 better’ than the U=0.4 window, but the U=0.1 Window is ‘5 better’ than the U=0.2 window.”
Yet, the incremental improvement is the same, Paulus states, adding that “an uneducated customer might be tempted to spend far more going from R-3 to R-5 than they would if they were looking at actual energy savings.”

I started this discussion by pointing out that by calling their program “the R-5 window volume purchase program,” DOE appears to be acknowledging the greater public awareness of R-values. They may not be advocating a change, however. Jerry Hartman of Gilkey Windows, one of the companies that has applied to participate in the R-5 program, points out that the requirements “are established using U values of .22 and less, so even the R-5 program is not switching to R-values.”

“After all the effort to get the window industry to stop talking about R-values and to use U-values, this certainly makes no sense,” says John Briggs of president of MidSouth Building Supply. He makes the same point as Hartman, and notes that to qualify as an R-5 product, double hung windows must have a U value of .22, which equates to R 4.54. Only the fixed units--which must have a .20 U-value--must really be R-5 products, he points out.

Gary Delman of Sunrise Windows suggests the industry should head back R-values, because they are better understood by the customer, but also cautions, "We can’t get tooooooooo hung up on any one thing."  

There are some windows out there which do in fact have low U values, which is a good thing, but "how good is it if that same window has a poor air infiltration rate?" he asks.  Suggesting that "we do need to develop a clear and concise way of communicating with the consumer so that they can understand what makes one product a better value than a competitive one," Delman says that Canadian approach may be "a tad" better than that used in the U.S.  "They calculate an ER (Energy Rating) number, and this ER is calculated by formula which takes into account a windows U value, its solar heat gain coefficient and its air infiltration rate. That way you get one number that gives you a 'better picture' of the products true energy performance," he asserts.

I don't know if want to open an ER debate in the U.S., but Delman highlights the real challenge–"a clear and concise way of communicating with the consumer." Given the split on this issue, something tells me it will be some time before we find the Holy Grail. 


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Should We Use R-Values? | Window & Door

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