[header] How to troubleshoot mold on interior walls [title] Examine areas with excessive moisture. [step] Check your home for sources of excessive moisture. These include flooding, water leaks, damp basements, hvac duct systems or shower or cooking steam. 
Look for uninsulated exterior windows and walls, including uninsulated closets along exterior walls where mold can grow when you run your heating in the winter. [substeps] Check the airflow in your home.