PDFWAC 51-11C-40391
Section C403.10—Construction of HVAC system elements.
C403.10 Construction of HVAC system elements. Ducts, plenums, piping and other elements that are part of an HVAC system shall be constructed and insulated in accordance with Sections C403.10.1 through C403.10.3.1.
C403.10.1 Duct and plenum insulation and sealing.
C403.10.1.1 Ducts, shafts, and plenums conveying outdoor air. Shafts and plenums conveying outdoor air from the exterior of the building to the mechanical system shall meet all air leakage and building envelope insulation requirements of Section C402, plus building envelope vapor control requirements from the International Building Code.
Ducts conveying outdoor air shall be insulated continuously from the building exterior to an automatic shutoff damper or heating or cooling equipment. Duct surfaces shall be insulated with the minimum insulation values in Table C403.10.1.1. Duct surfaces included as part of the building envelope shall not be used in the calculation of maximum glazing area as described in Section C402.4.1.
EXCEPTION: | Outdoor air ducts serving individual supply air units with less than 2,800 cfm of total supply air capacity, provided these are insulated to the minimum insulation values in Table C403.10.1.1. |
Table C403.10.1.1
Outdoor Air Ductwork Insulation
Duct system | Duct Location and Use | Climate Zone | Airflow | Minimum Installed Duct Insulation R-valuea,b | Notes |
Outdoor Air | Inside conditioned space and upstream of automatic shutoff damper | 4C and 5B | ≥ 2800 CFM | R-16 | See Section C403.10.1.1 for additional requirements |
Outdoor Air | Inside conditioned space and downstream of automatic shutoff damper to HVAC unit or room | 4C | ≥ 2800 CFM | R-8 | |
Outdoor Air | Inside conditioned space and downstream of automatic shutoff damper to HVAC unit or room | 5B | ≥ 2800 CFM | R-12 | |
Outdoor Air | Inside conditioned space | 4C and 5B | ≤ 2800 CFM | R-7 | See Exception 1 to Section C403.10.1.1 for additional details |
a | Insulation R-values, measured in h·ft2·°F/Btu, are for the insulation as installed and do not include film resistance. The required minimum thicknesses do not consider water vapor transmission and possible surface condensation. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75°F at the installed thickness. |
b | See International Mechanical Code Sections 603.12 and 604 for further details on duct insulation requirements. |
C403.10.1.2 Other supply and return ducts. All other supply and return air ducts and plenums shall be insulated with a minimum of R-6 insulation where located in unconditioned spaces, and where located outside the building with a minimum of R-8 insulation in Climate Zone 4 and R-12 insulation in Climate Zone 5. Ducts located underground beneath buildings shall be insulated as required in this section or have an equivalent thermal distribution efficiency. Underground ducts utilizing the thermal distribution efficiency method shall be listed and labeled to indicate the R-value equivalency. Where located within a building envelope assembly, the duct or plenum shall be separated from the building exterior or unconditioned or exempt spaces by minimum insulation value as required for exterior walls by Section C402.1.3.
EXCEPTIONS: | 1. Where located within equipment. |
2. Supply and return ductwork located in unconditioned spaces where the design temperature difference between the interior and exterior of the duct or plenum does not exceed 15°F (8°C) and are insulated in accordance with Table C403.10.1.2. |
Where located within conditioned space, supply ducts which convey supply air at temperatures less than 55°F or greater than 105°F shall be insulated with a minimum insulation R-value in accordance with Table C403.10.1.2.
EXCEPTION: | Ductwork exposed to view within a zone that serves that zone is not required to be insulated. |
Where located within conditioned space, return or exhaust air ducts that convey return or exhaust air downstream of an energy recovery media shall be insulated with a minimum insulation R-value in accordance with Table C403.10.1.2.
All ducts, air handlers, and filter boxes shall be sealed. Joints and seams shall comply with Section 603.9 of the International Mechanical Code.
Table C403.10.1.2
Supply, Return, Exhaust and Relief Air Ductwork Insulation
Duct System | Duct Location and Use | Climate Zone | Minimum Installed Duct Insulation R-valuea,b | Notes |
Supply air or return air | Outside the building (outdoors and exposed to weather)c | 4C | R-8 | See Section C403.10.1.2 for details |
Supply air or return air | Outside the building (outdoors and exposed to weather)c | 5B | R-12 | See Section C403.10.1.2 for details |
Supply air or return air | Unconditioned space (enclosed but not in the building conditioned envelope) | 4C and 5B | R-6 | See Section C403.10.1.2 for details |
Supply air or return air | Unconditioned space where the duct conveys air that is within 15°F of the air temperature of the surrounding unconditioned space | 4C and 5B | R-3.3 | See IMC Section 603.12 for additional requirements for condensation control at ductwork |
Supply air or return air | Where located in a building envelope assembly | 4C and 5B | R-16 | Duct or plenum is separated from building envelope assembly with the minimum insulation value |
Supply air | Within conditioned space where the supply duct conveys air that is less than 55°F or greater than 105°F | 4C and 5B | R-3.3 | See Section C403.10.1.2 for details |
Supply air | Within conditioned space that the duct directly serves where the supply duct conveys air that is less than 55°F or greater than 105°F | 4C and 5B | None | See Section C403.10.1.2 for details |
Supply air | Within conditioned space where the supply duct conveys air that is 55°F or greater and 105°F or less | 4C and 5B | None | |
Return or exhaust air | Within conditioned space, downstream of an energy recovery media, upstream of an automatic shutoff damper | 4C | R-8 | |
Return or exhaust air | Within conditioned space, downstream of an energy recovery media, upstream of an automatic shutoff damper | 5B | R-12 | |
Relief or exhaust air | Conditioned space and downstream of an automatic shutoff damper | 4C and 5B | R-16 |
a | Insulation R-values, measured in h·ft2·°F/Btu, are for the insulation as installed and do not include film resistance. The required minimum thicknesses do not consider water vapor transmission and possible surface condensation. Insulation resistance measured on a horizontal plane in accordance with ASTM C518 at a mean temperature of 75°F at the installed thickness. |
b | See International Mechanical Code Sections 603.12 and 604 for further details on duct insulation requirements. |
c | Includes attics above insulated ceilings, parking garages and crawl spaces. |
C403.10.2 Duct construction. Ductwork shall be constructed and erected in accordance with the International Mechanical Code. For the purposes of this section, longitudinal seams are joints oriented in the direction of airflow. Transverse joints are connections of two duct sections oriented perpendicular to airflow. Duct wall penetrations are openings made by any screw, fastener, pipe, rod, or wire. All other connections are considered transverse joints including, but not limited to, spin-ins, taps, and other branch connections, access door frames and jambs, and duct connections to equipment.
C403.10.2.1 Low-pressure duct systems. Longitudinal and transverse joints, seams and connections of supply and return ducts operating at a static pressure less than or equal to 2 inches water gauge (w.g.) (500 Pa) shall be securely fastened and sealed with welds, gaskets, mastics (adhesives), mastic-plus embedded-fabric systems or tapes installed in accordance with the manufacturer's installation instructions. Pressure classifications specific to the duct system shall be clearly indicated on the construction documents in accordance with the International Mechanical Code.
EXCEPTION: | Continuously welded and locking-type longitudinal joints and seams on ducts operating at static pressures less than 2 inches water gauge (w.g.) (500 Pa) pressure classification. |
C403.10.2.2 Medium-pressure duct systems. Ducts and plenums designed to operate at a static pressure greater than 2 inches water gauge (w.g.) (500 Pa) but less than 3 inches w.g. (750 Pa) shall be insulated and sealed in accordance with Section C403.10.1. Pressure classifications specific to the duct system shall be clearly indicated on the construction documents in accordance with the International Mechanical Code.
C403.10.2.3 High-pressure duct systems. Ducts designed to operate at static pressures equal to or greater than 3 inches water gauge (w.g.) (750 Pa) shall be insulated and sealed in accordance with Section C403.10.1. In addition, ducts and plenums shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual and shown to have a rate of air leakage (CL) less than or equal to 4.0 as determined in accordance with Equation 4-12.
(Equation 4-12)
CL | F/P0.65 | |
Where: | ||
F | The measured leakage rate in cfm per 100 square feet of duct surface. | |
P | The static pressure of the test. |
Documentation shall be furnished demonstrating that representative sections totaling at least 25 percent of the duct area have been tested and that all tested sections meet the requirements of this section.
C403.10.3 Piping insulation. All piping, other than field installed HVAC system refrigerant piping, serving as part of a heating or cooling system shall be thermally insulated in accordance with Table C403.10.3.
EXCEPTIONS: | 1. Factory-installed piping within HVAC equipment tested and rated in accordance with a test procedure referenced by this code. |
2. Factory-installed piping within room fan-coils and unit ventilators tested and rated according to AHRI 440 (except that the sampling and variation provisions of Section 6.5 shall not apply) and 840, respectively. | |
3. Piping that conveys fluids that have a design operating temperature range between 60°F (15°C) and 105°F (41°C). | |
4. Piping that conveys fluids that have not been heated or cooled through the use of fossil fuels or electric power. | |
5. Strainers, control valves, and balancing valves associated with piping 1 inch (25 mm) or less in diameter. | |
6. Direct buried piping that conveys fluids at or below 60°F (15°C). | |
7. In radiant heating systems, sections of piping intended by design to radiate heat. |
Table C403.10.3
Minimum Pipe Insulation Thickness (thickness in inches)a
Fluid Operating Temperature Range and Usage (°F) | Insulation Conductivity | Nominal Pipe or Tube Size (inches) | |||||
Conductivity Btu • in. /(h • ft2 • °F)b | Mean Rating Temperature, °F | < 1 | 1 to < 1-1/2 | 1-1/2 to < 4 | 4 to < 8 | ≥ 8 | |
˃ 350 | 0.32 - 0.34 | 250 | 4.5 | 5.0 | 5.0 | 5.0 | 5.0 |
251 - 350 | 0.29 - 0.32 | 200 | 3.0 | 4.0 | 4.5 | 4.5 | 4.5 |
201 - 250 | 0.27 - 0.30 | 150 | 2.5 | 2.5 | 2.5 | 3.0 | 3.0 |
141 - 200 | 0.25 - 0.29 | 125 | 1.5 | 1.5 | 2.0 | 2.0 | 2.0 |
105 - 140 | 0.21 - 0.28 | 100 | 1.0 | 1.0 | 1.5 | 1.5 | 1.5 |
40 - 60 | 0.21 - 0.27 | 75 | 0.5 | 0.5 | 1.0 | 1.0 | 1.0 |
< 40 | 0.20 - 0.26 | 75 | 0.5 | 1.0 | 1.0 | 1.0 | 1.5 |
a | For piping smaller than 1-1/2 inch (38 mm) and located in partitions within conditioned spaces, reduction of these thicknesses by 1 inch (25 mm) shall be permitted (before thickness adjustment required in footnote b) but not to a thickness less than 1 inch (25 mm). |
b | For insulation outside the stated conductivity range, the minimum thickness (T) shall be determined as follows: |
T | r{(1 + t/r)K/k - 1} | |||
Where: | ||||
T | = | Minimum insulation thickness. | ||
r | = | Actual outside radius of pipe. | ||
t | = | Insulation thickness listed in the table for applicable fluid temperature and pipe size. | ||
K | = | Conductivity of alternate material at mean rating temperature indicated for the applicable fluid temperature (Btu × in/h × ft2 × °F). | ||
k | = | The upper value of the conductivity range listed in the table for the applicable fluid temperature. | ||
c | For direct-buried heating and hot water system piping, reduction of these thicknesses by 1-1/2 inches (38 mm) shall be permitted (before thickness adjustment required in footnote b but not to thicknesses less than 1 inch (25 mm). |
C403.10.3.1 Protection of piping insulation. Piping insulation exposed to weather shall be protected from damage, including that due to sunlight, moisture, physical damage and wind, and shall provide shielding from solar radiation that can cause degradation of the material. Protection shall be removable for no less than six feet from the equipment for maintenance. Adhesive tape shall not be permitted.
C403.10.4 Insulation of HVAC system refrigerant piping. Field installed HVAC refrigerant piping, other than piping factory installed in HVAC equipment, shall have insulation as listed below, at a conductivity rating of 0.21 to 0.26 Btu × in/(h × ft2 × °F) with a mean temperature rating of 75°F. Piping insulation exposed to weather shall be protected from damage, including that due to sunlight, moisture, physical damage and wind, and shall provide shielding from solar radiation that can cause degradation of the material. Adhesive tape shall not be permitted. Manufacturer's required minimum pipe insulation shall be maintained.
1. For lines that convey hot gas for space heating:
1.1. Minimum 1-inch insulation on the portions outside the building thermal envelope.
1.2. Minimum 1/2-inch insulation on the portions within the building thermal envelope.
2. Minimum 1/2-inch insulation on the liquid line for mini-split systems and other systems for which insulation is required by the manufacturer, or where the metering device is located in the outdoor unit.
3. No insulation is required on the liquid line for other heat pump types or for cooling-only units where insulation is not required by the manufacturer.
[Statutory Authority: RCW 19.27A.020, 19.27A.025, 19.27A.160 and chapters 19.27A and 19.27 RCW. WSR 22-14-091, 23-12-101, and 23-20-021, § 51-11C-40391, filed 7/1/22, 6/7/23, and 9/25/23, effective 3/15/24. Statutory Authority: RCW 19.27A.025, 19.27A.045 and chapter 19.27 RCW. WSR 20-21-080, § 51-11C-40391, filed 10/19/20, effective 2/1/21. Statutory Authority: RCW 19.27A.020, 19.27A.025, 19.27A.160 and chapter 19.27 RCW. WSR 19-24-040, § 51-11C-40391, filed 11/26/19, effective 7/1/20.]