46 CFR §169.315

1. (a)All enclosed spaces within the vessel must be properly ventilated in a manner suitable for the purpose of the space.
2. (b)A means must be provided to close off all vents and ventilators.
3. (c)Living spaces must be ventilated by a mechanical system unless it can be shown that a natural system will provide adequate ventilation in all ordinary weather conditions. Provided that paragraph (a) of this section is satisfied, a vessel having only a natural ventilation system must satisfy the following: V/A≥1.4 where V is the total area of the vents in square inches and A is the product in square feet of the vessel's design waterline length times its maximum beam.