Does a bathroom need an exhaust fan under the IRC?

Work it through

You're the contractor. Pick what you'd do, then see how each choice plays out.

1. AIRC R303.3 requires mechanical ventilation (exhaust fan) in all bathrooms. A window can supplement but doesn't replace the fan requirement in most jurisdictions. Install the fan — it's code
   Correct

   IRC R303.3 requires bathrooms to have either a window (with specific minimum size) OR mechanical ventilation. Many local jurisdictions have amended this to require mechanical ventilation regardless of windows. During a renovation that includes permits, the inspector will check for adequate ventilation. A bathroom exhaust fan (50 CFM minimum for standard bathrooms) prevents moisture damage and mold. It costs $150 installed. Mold remediation costs $5,000+.

2. BThey have a window — that satisfies the ventilation requirement
   Check local code

   The IRC allows natural ventilation via an operable window with at least 4% of floor area. But many Maryland jurisdictions have adopted amendments requiring mechanical ventilation in all bathrooms, especially during renovations. Check YOUR jurisdiction's specific requirements before assuming a window alone is sufficient. Also consider: a window that 'cracks' open in January isn't providing ventilation.