Energy Code Compliance: Thermal Resistance of Air Spaces Behind Exterior Wall Coverings for Residential Applications

June 3rd, 2011 - Posted by RESNET under Insulation

R-values for air spaces behind residential wall claddings are sometimes used as a means to gain energy code compliance. However, the thermal resistance (R-value) assigned to such air spaces must be determined in a code-compliant manner. Since most residential cladding installations are air permeable and do not meet the requirements for an “ideal” air space, the energy code and Federal Trade Commission require air space R-values to be determined by testing the assembly as it will be constructed. For ideal air spaces as defined by ASHRAE Fundamentals, a code-compliant R-value can be selected from tables referenced in ASHRAE Fundamentals. However, ideal air spaces rarely exist behind air-permeable claddings used for residential construction. Recent testing has shown that the R-value of a reflective air space behind a typical air-permeable cladding, such as vinyl siding, provides as little as 15 percent of the R-value reported for ideal air spaces in ASHRAE Fundamentals. A more comprehensive version of this article can be found at the Foam Sheathing Coalition Website at http://www.foamsheathing.org/.

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