Plaster deteriorating around HVAC vents.

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The house is 1958 vintage in Eastcoast Central FL. HVAC was added later and all metal ductwork with metal ceiling vents were used. Plaster is crumbling about 2" around most ceiling vents. Is this probably caused by moisture? Will insulation fix it? What kind? 'Want to stop the problem before I repair/paint the plaster ceiling.