Ducts running through unconditioned attics or crawlspaces can create problems in all climates. Undesirable positive or negative pressurization, moisture and mold issues, and increased energy costs are often the result. In my work as an energy auditor, I’ve conducted blower door tests on many new homes that have duct work located outside the building envelope, many with poor test results.
As a matter of fact, of all the new homes I have tested that have failed code-required blower door tests, all had ducts in unconditioned attics or crawl spaces. With so many new slab-on-grade homes being built today, lots of designers and builders are trying to figure out: where is the best place to put the duct work? Before we get to that question, let’s look at some of what the 2018 International Residential Code (IRC) says about ducts outside of the building envelope.
What the IRC has to say about ducts
N1103.3 Ducts. Ducts and air handlers shall be in accordance with Sections N1103.3.1 through N1103.3.8.
N1103.3.1 Insulation (Prescriptive). Supply and return ducts in attics shall be insulated to an R-value of not less than R-8 for ducts 3 in. in diameter and larger and not less than R-6 for ducts smaller than 3 in. in diameter. Supply and return ducts in other portions of the building shall be insulated to not less than R-6 for ducts 3 inches in diameter and to not less than R4.2 for ducts smaller than 3 inches in diameter.
Exception: Ducts or portions thereof located completely inside the building thermal envelope.
N1103.2.2 Sealing (Mandatory). Ducts, air handlers, and filter boxes shall be sealed. Joints and seams shall comply with section M1601.4.1 of this code.
N1184.108.40.206 Sealed air handler. Air handlers shall have a manufacturer’s designation for an air leakage of not greater than 2% of the…
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