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Why R is Not Simply the Inverse of U - aamanet.org

www.dwmmag.com d o o r & W I N D O W M A N U F A C T U R E R 10 AAMA ANALYSIS Why R is Not Simply the Inverse of U BY KEN BRENDEN Q uick question: which is better—something rated 0.2


  Inverses, Simply, Is not simply the inverse of u




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Text of Why R is Not Simply the Inverse of U - aamanet.org

ANALYSISWhy R is Not Simply the Inverse of UBY KEN BRENDENQuick question: which is better something rated 5?When the Department of Energyintroduced its R-5 window pro-gram, some folks in the windowindustry might have done a double-take. After all, window thermal per-formance rating systems havealways used the term U-value or U-factor to define window thermalperformance. The latter, in fact, isclearly emblazoned on the NationalFenestration Rating Council (NFRC)thermal performance label. So, whythe sudden reference to R-value? Many consumers have heard of R-values a measure of a product sresistance to heat flow and easilygrasp that materials with higher R-values, such as R-19 vs. R-11 wallinsulation, are more energy-effi-cient. That is, the better the productis at stopping the flow of heat to theoutside during winter or to theinside during summer, the easier(and cheaper) it is for HVAC systemsto maintain the desired indoor tem-perature for a longer , an R-value is given for acertain thickness of a specifichomogenous material such as fiber-glass insulation. The R-value of astructure that is made of layers ofdifferent materials (exterior sheath-ing, insulation, interior dry wall, etc.)can be estimated simply by addingthe R-values of the individual layers. While R-value measures resist-ance to heat transfer, U-valuemeasures the rate of heat lower its U-value, the betterthe product s ability to resist heatconduction. In simple terms, U-value is themathematical reciprocal of R-value;that is, U = 1/R and R = 1/U. Forinstance, a material with an R-Valueof 5 has a U-value of (1 divided by5). As the R-value goes up, the U-value goes down, and , it actually is more compli-cated than officially as thermal trans-mittance, U-value is more of anengineering term used to designatethe thermal performance of a systemas opposed to that of a homoge-neous material. U-value thus hasbeen used traditionally to expressthe thermal efficiency of windows,which, unlike wall insulation, arecomplex assemblies of componentswith a variety of sometimes conflict-ing missions. Wall insulation is singularlyintended to limit heat transfer, mak-ing it fairly straightforward to assessinsulation strictly on its ability toachieve that objective. However,windows may need to permit venti-lation while limiting unwanted airinfiltration and admit daylight whileoptimizing the effects of solar heatgain. Since R-value currently isapplied to homogenous materialswith a singular purpose and isunderstood in the marketplacebased on such products, it may notbe equitably applicable to multi-purpose products with varying con-struction, such as windows. U-value accounts for how energyenters and leaves the material; itconsiders both conduction andradiation. R-value accounts onlyfor the resistance to heat flow byconduction. Per NFRC 100-2010, Proceduresfor Determining FenestrationProduct U-Factors, the overall U-value of a window product is a pro-rated summation of U-values of thecenter of glass, edge of glass andframe areas. This takes into accountsuch details as insulating glass edgespacers, certain hardware and , R-value is used todefine the energy efficiency ofmany building materials because itis intuitively easier for consumersto understand that R-19 insulationis better than R-11, rather than try-ing to explain how U= insula-tion is better than U= , in the case of windows,using U-values avoids comparing awindow s multi-faceted overall per-formance to the purely insulatingvalue of a wall. So, while it is unusual to see win-dow energy efficiency expressed interms of R-value, it does relate well tothe average consumer s understand-ing of insulating capability. However,it also could lead to confusion sinceR-value doesn t take into account allof the facets of a fenestration prod-uct the way U-value does. (For moreon U- versus R-values, see July-August DWM, page 21.) Ken Brenden serves as technical standardsmanager for the American ArchitecturalManufacturers Association inSchaumburg, Ill. He may be reached His opinions aresolely his own and do not necessarilyreflect those of this the R-value goes up, theU-value goes down, andvice-versa. However, it actually is morecomplicated than that. 2010 Door & Window Manufacturer Magazine, 540-720-5584, , All rights reserved. Reprinted with permission.

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