As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 5228 - ON CLIMBOUT FROM CVG RECEIVED CABIN ALTITUDE WARNING MESSAGE, WITH PACKS ON, CABIN PRESS RISING. AIRCRAFT RETURNED TO CVG AND LANDED SAFELY. UPON INSPECTION, FOUND A 16" SQUARE PIECE OF INSULATION PROJECTING THROUGH PRIMARY OUTFLOW VALVE. REMOVED INSULATION FROM VALVE AND RESECURED BACK IN PLACE ON FUSELAGE. NO FURTHER DEFECTS.