46 C.F.R. § 108.455 – Enclosure openings.

(a) Mechanical ventilation for spaces protected by a CO2 system must be designed to shut down automatically when the system is activated.
(b) Each space that is protected by a CO2 system and that has natural ventilation must have a means for closing that ventilation.
(c) Each space protected by a CO2 system must have the following means for closing the openings to the space from outside the space:
1. (1) Doors, shutters, or dampers for closing each opening in the lower portion of the space.
2. (2) Doors, shutters, dampers or temporary means such as canvas or other material normally on board a unit may be used for closing each opening in the upper portion of the space.