4.3.1.3.1 Gypsum board having a maximum flame spread index of 25 without evidence of
continued progressive combustion and a maximum smoke developed index of 50 when tested
in accordance with ASTM E 84, Standard Test Method for Surface Burning Characteristics of
Building Materials, or ANSI/UL 723, Standard Test Method for Surface Burning
Characteristics of Building Materials, shall be permitted to be used for negative pressure
exhaust and return ducts where the temperature of the conveyed air does not exceed 52°C
(125°F) in normal service
4.3.1.3.2 The air temperature limits of 4.3.1.3.1 shall not apply when gypsum board material is
used for emergency smoke exhaust air ducts.
4.3.1.4 All air duct materials shall be suitable for continuous exposure to the temperature and
humidity conditions of the environmental air in the air duct.
4.3.1.5 The materials, thickness, construction, and installation of ducts shall provide structural
strength and durability in conformance with recognized good practice.
4.3.1.5.1 Air ducts shall be considered to be in compliance with this requirement where
constructed and installed in accordance with the ASHRAE Handbook — HVAC Systems and
Equipment, and with one of the following as applicable:
4.3.1.3.1 Gypsum board having a maximum flame spread index of 25 without evidence of
continued progressive combustion and a maximum smoke developed index of 50 when tested
in accordance with ASTM E 84, Standard Test Method for Surface Burning Characteristics of
Building Materials, or ANSI/UL 723, Standard Test Method for Surface Burning
Characteristics of Building Materials, shall be permitted to be used for negative pressure
exhaust and return ducts where the temperature of the conveyed air does not exceed 52°C
(125°F) in normal service
4.3.1.3.2 The air temperature limits of 4.3.1.3.1 shall not apply when gypsum board material is
used for emergency smoke exhaust air ducts.
4.3.1.4 All air duct materials shall be suitable for continuous exposure to the temperature and
humidity conditions of the environmental air in the air duct.
4.3.1.5 The materials, thickness, construction, and installation of ducts shall provide structural
strength and durability in conformance with recognized good practice.
4.3.1.5.1 Air ducts shall be considered to be in compliance with this requirement where
constructed and installed in accordance with the ASHRAE Handbook — HVAC Systems and
Equipment, and with one of the following as applicable:
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