© Copyright CalCERTS, Inc. 2019
Code Cycle: 2016 & 2019 Standards
Reference: CalCERTS Alterations Training Manual Chapter 7, Section 5
Question: Can a toe kick area under a cabinet be used as a plenum to supply air conditioning, or does the supply require ducting?
Answer: Mandatory Measures as found in 150.0(m)1 does not allow building cavities to be used for conveying conditioned air. The following excerpt is from the 2019 Standards. The highlighted paragraph is also found in the 2013 and 2016 Standards:
150.0(m) - Air-Distribution and Ventilation System Ducts, Plenums, and Fans.
1. CMC Compliance.
A. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, 605.0 and
ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference.
B. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall either be insulated to:
i. a minimum installed level of R-6.0, or
ii. a minimum installed level of R-4.2 when the duct system is located entirely in conditioned space as
confirmed through field verification and diagnostic testing in accordance with the requirements of
Reference Residential Appendix RA220.127.116.11.8.
EXCEPTION 1 to Section 150.0(m)1B: Portions of the duct system located in wall cavities are not
required to be insulated if the following conditions are met:
i. The cavity, duct or plenum is located entirely inside the building’s thermal envelope as confirmed
by visual inspection.
ii. At all locations where portions of non-insulated cavities, ducts, or plenums make a transition into
unconditioned space, the transition shall be air-sealed to prevent air infiltration into the cavity and
be insulated to a minimum of R-6 as confirmed by visual inspection.
EXCEPTION 2 to Section 150.0(m)1B: Portions of the duct system completely exposed and
surrounded by directly conditioned space are not required to be insulated.
C. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened.
D. Openings shall be sealed with mastic, tape, or other duct-closure system that meets the applicable
requirements of UL 181, UL 181A or UL 181B or aerosol sealant that meets the requirements of UL
723. If mastic or tape is used to seal openings greater than 1/4 inch, the combination of mastic and
either mesh or tape shall be used.
E. Building cavities, support platforms for air handlers, and plenums designed or constructed with
materials other than sealed sheet metal, duct board or flexible duct shall not be used for conveying
conditioned air. Building cavities and support platforms may contain ducts. Ducts installed in cavities
and support platforms shall not be compressed to cause reductions in the cross-sectional area of the
EXCEPTION to Section 150.0(m)1: Ducts and fans integral to a wood heater or fireplace.
For new construction, a toe kick cannot be used for conveying conditioned air, unless it were designed with the approved materials.
For alterations that already have toe kick supplies, the duct leakage would be tested at 15%. If it did not comply, then a smoke test and seal "all accessible" leaks could be used.