International Code Council Standards& Building Code Standards
The International Code Council (ICC) is the standard by which all states and local Building Codes follow in residential new construction and existing residential retrofits. The code states that all bathroom exhaust fans must vent to the exterior of a structure. Venting bathroom exhaust fans to the exterior of a structure is required to displace moisture out of the structure. Effective moisture displacement to the exterior of a structure subscribes to effective removal as well as inhibiting reentry into the structure. Effective displacement of moisture out of a structure helps prevent mold growth, structural damage, and potential health issues related to poor indoor air quality.
State and local building codes follow the ICC standards for bathroom exhaust venting. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space. The standards are found in Chapter 15: Exhaust Systems, Section M1501 of the ICC Standard of Codes.
“Exhaust air from bathrooms and toilet rooms shall not be re-circulated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space, or other areas inside the building.”