What was found
RNO - FLIGHT 2516, AT CRUISE FLIGHT AT 38,000 FEET RECEIVED A MASTER CAUTION MESSAGE FOR THE CABIN ALTITUDE. CABIN ALTITUDE CONTINUED TO CLIMB TO 11,400. THE PASSENGER OXYGEN MASKS DEPLOYED AND STARTED A DESCENT TO A SAFE ALTITUDE. DIVERTED TO RENO AND ACCOMPLISHED A UNEVENTFUL LANDING. MAINTENANCE FOUND THE NR 1 SAFETY OUTFLOW VALVE HAD LOOSE INSULATION AROUND IT KEEPING THE VALVE IN THE OPEN POSITION. CLEARED INSULATION FROM AREA. PERFORMED A OPERATIONAL CHECK OF THE PRESSURIZATION SYSTEM. CHECKS GOOD WITH NO DEFECTS. AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.