The vapour barrier is the job
On chilled ducts and pipes, the insulation keeps the surface warm and the vapour barrier keeps humid air away from the cold metal. A torn facing, an unsealed joint or a hanger that pierces the barrier lets moisture in, which condenses, soaks the insulation and drips through the ceiling. In the humid coastal cities of Saudi Arabia, this is the most common insulation defect. Good insulators treat every joint, hanger and penetration as part of the barrier.
