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CBO
A recent code interpretation provides critical guidance on whether exhaust ducts are permitted to penetrate fire-resistance-rated horizontal assemblies in multi-family residential buildings. The clarification applies to the 2018, 2021, and 2024 editions of the International Building Code (IBC) and addresses the routing of bath and clothes dryer exhaust ducts through a one-hour rated floor/ceiling assembly in a two-story, Type VB apartment building.
The key provision is IBC Section 717.6.2, which generally requires that ducts penetrating the ceiling membrane of a fire-resistance-rated floor/ceiling assembly be protected with either a shaft enclosure or a ceiling radiation damper. However, there are specific exceptions to this requirement. Notably, Exception 2 of Item 2 permits such penetrations when the duct originates within the dwelling unit and the penetration is protected with a firestop system in accordance with Section 714.5.1.2.
In the case at hand, the exhaust ducts are routed from the bathroom and laundry area up through an interior wall cavity and penetrate the rated floor assembly at the top plate. Once within the floor truss space (the rated horizontal assembly), the ducts run laterally to terminate at a sidewall. According to the ICC’s interpretation, this configuration is permitted—provided that the penetration is firestopped using a tested and listed system that complies with ASTM E814 or UL 1479, as required by Section 714.5.1.2.
The 2018 International Mechanical Code (IMC) Section 504.2 is also relevant. It prohibits the installation of ceiling radiation dampers in dryer exhaust ducts. This creates a conflict unless Exception 2 of Section 717.6.2 is used, which does not require a damper if the penetration is properly firestopped.
Additionally, there is no requirement in the code to install fire-resistive wrapping around the ducts once they are within the horizontal cavity—as long as the ducts remain within the same dwelling unit and do not pass through another unit. In this interpretation, running the exhaust laterally through the ceiling space to reach an exterior wall is considered an extension within the same unit, even if there is a dwelling above, because the ducts do not enter that upper unit’s occupied space.
Conclusion:
Under Exception 2 of Section 717.6.2, and in compliance with Section 714.5.1.2, the proposed method of routing dryer and bath exhaust ducts through the ceiling membrane of a one-hour rated floor/ceiling assembly is code-compliant. Ceiling radiation dampers are not required in this case, and no additional fire-rated duct wrapping is necessary once inside the ceiling cavity, assuming the ducts remain within the originating dwelling unit.
The key provision is IBC Section 717.6.2, which generally requires that ducts penetrating the ceiling membrane of a fire-resistance-rated floor/ceiling assembly be protected with either a shaft enclosure or a ceiling radiation damper. However, there are specific exceptions to this requirement. Notably, Exception 2 of Item 2 permits such penetrations when the duct originates within the dwelling unit and the penetration is protected with a firestop system in accordance with Section 714.5.1.2.
In the case at hand, the exhaust ducts are routed from the bathroom and laundry area up through an interior wall cavity and penetrate the rated floor assembly at the top plate. Once within the floor truss space (the rated horizontal assembly), the ducts run laterally to terminate at a sidewall. According to the ICC’s interpretation, this configuration is permitted—provided that the penetration is firestopped using a tested and listed system that complies with ASTM E814 or UL 1479, as required by Section 714.5.1.2.
The 2018 International Mechanical Code (IMC) Section 504.2 is also relevant. It prohibits the installation of ceiling radiation dampers in dryer exhaust ducts. This creates a conflict unless Exception 2 of Section 717.6.2 is used, which does not require a damper if the penetration is properly firestopped.
Additionally, there is no requirement in the code to install fire-resistive wrapping around the ducts once they are within the horizontal cavity—as long as the ducts remain within the same dwelling unit and do not pass through another unit. In this interpretation, running the exhaust laterally through the ceiling space to reach an exterior wall is considered an extension within the same unit, even if there is a dwelling above, because the ducts do not enter that upper unit’s occupied space.
Conclusion:
Under Exception 2 of Section 717.6.2, and in compliance with Section 714.5.1.2, the proposed method of routing dryer and bath exhaust ducts through the ceiling membrane of a one-hour rated floor/ceiling assembly is code-compliant. Ceiling radiation dampers are not required in this case, and no additional fire-rated duct wrapping is necessary once inside the ceiling cavity, assuming the ducts remain within the originating dwelling unit.
