From site observations, the following factors contribute extensively to water condensation problems.
Plenum boxes invariably are installed without external insulation resulting eventually in water dripping downwards to the grilles and ceiling. Closed-cell rubber external insulation is strongly recommended in addition to internal fibreglass especially where the ceiling space has high temperature and RH factors. Our experience shows that a 13mm closed-cell rubber insulation is superior to 50mm fibreglass in extreme environments.
Air leakage in punctured flexible ducts have resulted in condensation at ceiling surfaces or water accumulation in such ducts; the accumulated water eventually drains to the grilles.
Air leakage from improperly installed grilles or defective grille designs exacerbate problems with resultant condensation at grilles and ceiling surfaces.