506.11 Clearance Reduction Methods — 2025 California Mechanical Code
California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Mechanical Code (Title 24, Part 4) § 506.11 · Effective January 1, 2026 · Official source: Building Standards Commission
It shall be permitted to protect stored combustible material or combustible construction from ductwork in accordance with Table 506.11 and Section 506.11.1 through Section 506.11.6. In no case shall the clearance between the duct and the combustible surface be reduced below that allowed in Table 506.11. [NFPA 91:4.7.4, 4.7.4.1] 506.11.1 Spacers and Ties. Spacers and ties for protection materials shall be of noncombustible material and shall not be installed on the duct side of the protection system. [NFPA 91:4.7.4.2]
506.11.2 Wool Batts Insulation. Mineral wool batts (blanket or board) shall have a minimum density of 8 pounds per cubic feet (lb/ft [3] ) (128 kg/m [3] ) and have a minimum melting point of 1500°F (816°C). [NFPA 91:4.7.4.3]
506.11.3 Insulation Board. Insulation board used as a part of a clearance-reduction system shall meet the following criteria:
(1) Have a thermal conductivity of 1 British thermal unit inch per hour square foot degree Fahrenheit
[Btu•in/(h•ft [2] - °F)] [0.14 W/(m•K)] or less.
(2) Be formed of noncombustible material. [NFPA 91:4.7.4.4]
506.11.4 Duct and Thermal Shield. With all clearance reduction systems, at least 1 inch (25.4 mm) clear space shall be provided between the duct and the thermal shield. [NFPA 91:4.7.4.5]
506.11.5 Thermal Shield and Combustible Sur- face. When using clearance reduction systems that include an air gap, at least 1 inch (25.4 mm) clear space shall be provided between the thermal shield and the combustible surface. [NFPA 91:4.7.4.6]
506.11.6 Reduced Clearance with Air Gaps. When using clearance reduction systems that include an air gap between the combustible surface and the selected means of protection, air circulation shall be provided by one of the methods in Section 506.11.6.1 through Section 506.11.6.3. [NFPA 91:4.7.4.7]
506.11.6.1 Air Circulation. Air circulation shall be permitted to be provided by leaving all edges of the protecting system open with at least a 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.1] 506.11.6.2 Single Flat Wall. If the means of protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one of the following:
(1) Leaving only the top and bottom edges open to circulation by maintaining the 1 inch (25.4 mm) air gap.
(2) Leaving the top and both side edges open to circulation by maintaining the 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.2] 506.11.6.3 Thermal Shielding. Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with at least 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.3]
TABLE 506.11
REDUCTION OF DUCT CLEARANCE WITH SPECIFIED FORMS OF PROTECTION
[NFPA 91: TABLE 4.7.4]
| FORM OF PROTECTION* | MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent) | |
|---|---|---|
| FORM OF PROTECTION* | AS WALL PROTECTOR OR VERTICAL SURFACE |
AS CEILING PROTECTOR OR HORIZONTAL SURFACE |
| 31⁄2 inch thick masonry wall without ventilated air space | 33 | None |
| 1⁄2 inch thick noncombustible insulation board over 1 inch glass fiber or min- eral wool batts without ventilated air space |
50 | 33 |
| 0.024 inch (24 gauge) sheet metal over 1 inch glass fiber or mineral wool batts reinforced with wire, or equivalent on rear face with at least a 1 inch air gap |
66 | 66 |
| 31⁄2 inch (90 mm) thick masonry wall with at least a 1 inch air gap | 66 | None |
| 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap | 66 | 50 |
| 1⁄2 inch thick noncombustible insulation board with at least a 1 inch air gap | 66 | 50 |
| 0.024 inch (24 gauge) sheet metal with ventilated air space over at least 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap |
66 | 50 |
| 1 inch glass fiber or mineral wool batts sandwiched between two sheets of 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap |
66 | 50 |
For SI units: 1 inch = 25.4 mm
- Clearance reduction applied to and covering all combustible surfaces within the distance specified as required clearance with no protection in Table 506.10.4.
108 2025 CALIFORNIA MECHANICAL CODE
EXHAUST SYSTEMS
Part II - Commercial Hoods and Kitchen
Ventilation.
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