The chilled-water riser pipes in our building's shaft are sweating and dripping — is the insulation wrong, and whose responsibility is it?

Almost certainly condensation, not a leak — and it's a classic riser problem. The chilled water inside those pipes is well below the shaft air's dew point, so wherever the insulation is missing, thin, wrong-type or unsealed, humid air hits cold metal and water forms and runs down. In a hot, unventilated riser shaft the humidity is high, so any weak spot sweats hard.

Where it goes wrong (in order): - The insulation type. Riser lagging must be closed-cell (see closed-cell vs open-cell) — if someone used an absorbent/open-cell material it soaks up condensation, collapses its R-value and drips permanently. - The vapour seal. Pipe insulation only works as a continuous, sealed vapour barrier — every joint, seam and end taped/glued. One open seam and moisture tracks under the insulation and sweats on the pipe (and quietly corrodes it). - The bits people skip — valves, flanges, supports and the pipe where it passes through floor slabs. Bare valves and uninsulated supports are the number-one dripping points. - Thickness — too thin for the pipe temperature and it still sweats.

Fix: re-lag in correctly-sized closed-cell elastomeric insulation (Armacell / K-Flex type), sealed throughout, including the valves and supports, and check for any pipe corrosion hiding under the old wet lagging.

Responsibility: the common vertical risers in a shared shaft are normally the building's / owners'-association responsibility (common property), while the pipework inside your own unit is yours — but check your building's specific documents. Either way it needs proper AC pipe insulation done as a sealed system, not a patch, or it comes straight back.