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DUCT DESIGN
3.5 Thermal Performance
Flexible Duct Media (FDM) is used to resist the flow of energy in insulated flexible air duct. Heat flows into the duct during cooling or out of the duct during heating. The quantity of heat flowing into or out of the duct is controlled by the FDM ’ s thermal resistance — commonly called R - value. Requirements for air distributing duct R - values vary depending on the energy code being enforced. The minimum R - value in conformance with this standard is R - 4.2 [0.74]. To accommodate these requirements, ADC manufacturers insulated flexible duct with R - value ratings of R4.2 [0.74], 6.0 [1.06], and 8.0 [1.41]. Special applications may call for R - values beyond 8.0. Uninsulated ducts are considered to have no R - value rating. Flexible duct R - values shall be based on insulation only and not include air barriers, air films, vapor barrier, or other ducting components.
Some helpful equations and terms related to Thermal Resistance, Thermal Conductance, and Thermal Conductivity are listed below. Heat Transfer is always from the hot side to the cold side of the insulation.
3.5.1 Thermal Conductivity (k and λ )
Thermal conductivity is a material property of FDM and the measure of its ability to conduct heat flowing through unit thickness per unit area per degree of temperature. It is often abbreviated as k in IPS units or λ n SI units.
3.5.2 Thermal Conductance (C - value)
Thermal Conductance is the measure of the ability of the FDM to conduct heat flowing through an actual FDM thickness per unit area per degree of temperature.
Equation 1: C - value = (k or λ ) / (thickness of FDM when laid flat at room temperature conditions)
3.5.3 Thermal Resistance (R - value)
Thermal Resistance is the measure of the ability of the FDM to resist heat flowing through an actual FDM thickness.
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Equation 2: R - value = 1 / C - value
Equation 3: R - value = (thickness of FDM when laid flat at room temperature conditions) / (k or λ )
3.6 Thermal Performance Certification
ADC maintains a third - party verification and certification protocol for flexible duct R - value ratings in accordance with Sections 6.2.1 and 6.6.1 of this standard.
A testing and follow - up program was established with UL and ETL in 1993 for flexible ducts listed and labeled to the UL181 Standard. This program includes testing at the flexible duct and the insulation manufacturer ’ s locations to classify component materials and flexible ducts. Continued follow - up testing at the factories insures continued R - Value classification. The program test method measures the thermal conductivity (k) value for the insulation at the intended duct wall thickness and verify the flexible duct components are manufactured to provide the required wall thickness to achieve the R - Value. Thermal performance ratings are based on the conductivity value for the insulation when tested in a flat configuration and do not include air barriers, air films, vapor barrier, or other ducting components. Most building codes emphasize the above requirements for determining duct R - Values and include language indicating duct insulation R - Values shall be based on the insulation and the installed wall thickness only. Look for the ADC Seal of Certification for Thermal Performance.
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