Vapour barrier

A vapour barrier is **a continuous, sealed layer that stops water *vapour* — not just liquid water — from reaching a cold surface. It's the part of AC and chilled-pipe insulation people most often get wrong, and in humid Dubai it's decisive. Here's why: a cold pipe or duct sits below the dew point, and humid air doesn't need a hole to cause trouble — water vapour diffuses right through any unsealed seam, joint or gap in the lagging**, reaches the cold metal, and condenses *inside* the insulation. You don't see it until it's dripping through the ceiling or the pipe has quietly corroded under wet lagging. So insulation only works when it's installed as an unbroken vapour barrier — every seam and butt-joint glued/taped, ends sealed, and no compression. This is exactly why cold pipes use sealed closed-cell lagging: the closed-cell material *is* the vapour barrier when it's properly sealed. The classic failure is a neat-looking job with open seams that sweats from day one. Getting the vapour seal continuous — valves and fittings included — is the difference between insulation that lasts and a call-back, which is the point of doing AC pipe insulation as a sealed system.