© Copyright CalCERTS, Inc. 2019
Code Cycle: 2019 Non-Residential Standards
Reference: CalCERTS Non-Residential Training Manual Chapter 4, Section 1
Form: NRCV-MCH-04 (Duct Leakage)
Duct Leakage Measure
140.4(L) - Air Distribution System Duct Leakage Sealing. Duct systems shall be sealed in accordance with 1 or 2 below:
1. Systems serving high-rise residential buildings, hotel/motel buildings and nonresidential buildings other than healthcare facilities, the duct system shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through field verification and diagnostic testing, in
accordance with the applicable procedures in Reference Nonresidential Appendices NA1 and NA2 if the
criteria in Subsections A, B and C below are met:
A. The duct system provides conditioned air to an occupiable space for a constant volume, single zone,
space-conditioning system; and
B. The space conditioning system serves less than 5,000 square feet of conditioned floor area; and
C. The combined surface area of the ducts located in the following spaces is more than 25 percent of the total surface area of the entire duct system:
i. Outdoors; or
ii. In a space directly under a roof that
a. Has a U-factor greater than the U-factor of the ceiling, or if the roof does not meet the
requirements of Section 140.3(a)1B, or
b. Has fixed vents or openings to the outside or unconditioned spaces; or
iii. In an unconditioned crawlspace; or
iv. In other unconditioned spaces.
2. Duct systems serving healthcare facilities shall be sealed in accordance with the California Mechanical
Remember, all of the three criteria listed, A, B, & C, must be met to require HERS duct leakage and sealing verification. If any of the three criteria are NOT fully met, then the duct system is exempt from testing.
The most common reason that duct leakage testing is exempted from a nonresidential project is due to duct location as per item C above. If no more than 25% of the ducts are located in unconditioned space, such as on the roof or in a vented attic or crawlspace, then the duct system is exempt from testing requirements. More than 25% of ducts located in unconditioned space triggers HERS verification.
The majority of modern nonresidential buildings do not have vented attics or crawlspaces. They typically have a T-bar ceiling design, which is a suspended ceiling with a metal T-bar grid that supports removable panels for accessing the space between the T-bar ceiling and the underside of the roof deck. Within that space, unsightly ducting, electrical, plumbing, fire sprinkler systems or other building systems are concealed.
In this design, the envelope insulation, serving as the thermal barrier, is typically attached to the underside of the roof sheathing, but there could also be rigid insulation above the roof deck. Ducts in this area between the insulated roof and the uninsulated T-bar ceiling are located within the thermal envelope of the building and are considered to be in “indirectly conditioned space.” Therefore, the duct system is exempt from duct leakage verification providing that no more than 25% of the ducts are located in unconditioned space, such as on a roof.
Note that for new construction projects, insulation is not typically installed on the T-bar ceiling panels because the numerous joints cannot be sealed and the assembly cannot serve as an air barrier. Thus, insulation installed on a T-bar ceiling is typically not recognized as a component of the building envelope and must be ignored as an insulation component.