CTL OF CABIN PRESSURIZATION IS LOST WHEN LOOSE MATERIAL BECOMES LODGED IN THE OUTFLOW VALVE.

Narrative

CRUISING AT 37000 FT; WHERE IT HAD BEEN ASSIGNED FOR SOME 2 HRS; JUST N OF DCA; FLC NOTICED AN ACUTE CHANGE OF CABIN PRESSURE MANIFESTED BY EAR DISCOMFORT. CREW IMMEDIATELY NOTICED CABIN RATE OF CLB 'PEGGED' AT 4000 FPM AND CABIN ALT APCHING 10000 FT. CREW DONNED OXYGEN MASKS AND ESTABLISHED COM WITH EACH OTHER. NO AURAL OR LIGHT WARNINGS WERE OBSERVED. FO MANUALLY SELECTED STANDBY MODE OF CABIN PRESSURIZATION WITH NO NOTICEABLE EFFECT ON THE CABIN'S ASCENT. FO THEN SELECTED MANUAL MODE AND USING THE DC SYSTEM TOGGLED THE OUTFLOW VALVE TOWARD THE CLOSED POS. CABIN PRESSURE WAS STABILIZED JUST UNDER 10000 FT AND THEN REPRESSURIZED TO APPROX 8000 FT. APPROPRIATE QUICK REACTION CHKLIST WAS THEN INITIATED AND PROCEEDED WITHOUT INCIDENT UNTIL THE AIR CONDITIONING PACKS WERE SWITCHED TO 'HIGH.' AT THIS POINT THE CABIN AGAIN STARTED ASCENDING AT 4000 FPM AND TOGGLING THE OUTFLOW VALVE TOWARD CLOSED SEEMED TO HAVE NO EFFECT ON THE CABIN RATE OF CLB. THE CREW SURMISED THE CABIN PRESSURE WAS UNCTLABLE AND ELECTED TO MAKE AN EMER DSCNT. DECLARED AN EMER TO ZDC AND SELECTED XPONDER TO 7700. CTR RESPONDED IMMEDIATELY WITH CLRNC TO 28000 FT. OTHER LOWER ALTS WERE ISSUED IN A TIMELY MANNER BY CTR THUS NEGATING DSNDING TO AN UNCLRED ALT. FLT REQUESTED A RADAR VECTOR TO DULLES; THE CLOSEST COMPANY ARPT NOT INUNDATED WITH TFC. DIVERSION WAS SELECTED DUE TO ACFT STRUCTURE AND FUEL CONSIDERATIONS. LNDG AT DULLES WAS ROUTINE. UPON INSPECTING THE ACFT NOTHING ABNORMAL WAS DISCOVERED EXCEPT A 3 FT SQUARE PIECE OF FIBERGLASS DUCT INSULATION LODGED IN THE MAIN (AFT) OUTFLOW VALVE. INSULATION WAS REMOVED AND NORMAL VALVE OP OBSERVED. FLT CONTINUED ON TO ORIGINAL DEST AFTER SECURING NECESSARY RELEASE FROM FLT CTL. THE ACFT WAS GIVEN A COMPLETE PRESSURIZATION CHK AT NEWARK AND FINDING NO DISCREPANCIES WAS RELEASED FOR SVC.