True R-Values of Round Residential Ductwork

Larry Palmiter and Erin Kruse, Ecotope, Inc.

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Abstract

It is a common misconception that the actual R-value of flexible duct is similar to the rated R-value. This is approximately true for lower nominal R-values. For instance, under typical conditions, a six-inch diameter, nominal R-4.2 duct has an actual R-value of 3.41 not counting film resistances. In this case, including the film resistances more than compensates for this loss, with the total R-value being 4.34. The combination of both factors does not always result in an improved total R-value however, and the discrepancy between nominal and actual R-value gets larger with higher R-values and/or smaller diameters. For example, a six-inch diameter, nominal R-8 flex duct has an actual R-value of only 5.62 not counting surface films. With the film resistances the total R-value of 6.45 is 19% less than the nominal value. For R-11 ducts (six-inch diameter) the R-value with films is only 7.81, 29% less than the nominal value. The primary cause for this discrepancy is that the rated R-value is measured with the insulation flat. When in use, it is wrapped in a cylinder around the duct, resulting in a reduced R-value. This paper reviews the basic concepts and equations for correct calculation of heat loss in a cylindrical geometry including the effects of diameter and insulation thickness and density. The effects of duct air velocity, temperature and density on the internal film coefficient are given, as well as a discussion on the effect that air density changes due to elevation have on duct efficiency. Tables are given to simplify the estimation of actual R-values.