Why do AC and chilled-water pipes drip, and how do you fix it?

6 min read · Written by our team · Updated 26 Sept 2026

A bare pale-blue cold pipe covered in dripping water droplets below a red-outlined ceiling stain, above a dry pipe covered in dark grey insulation.

In short

Water drips form when the pipe or insulation surface is colder than the dew point of the surrounding air, so water vapour condenses. Common causes are sections with no insulation, joints that were never glued or taped, insulation that is torn or crushed, or insulation that is not enough for the temperature and humidity of the space. The fix is to find the actual cause, then replace or add insulation suited to the real conditions and seal the joints so the system is continuous.

1The surrounding air holds water vapour; on a surface colder than its dew point, the vapour condenses into water
2Water collects on the pipe and drips onto the ceiling or floor
3Insulation sized for the pipe temperature and the site's temperature and humidity keeps its outer surface warmer than the dew point
4Seams must be sealed as the manufacturer specifies — through any gap, humid air can reach the pipe and condense under the insulation

Why do cold pipes "sweat"?

Air always contains water vapour. When it touches a surface colder than its dew point, the vapour condenses into droplets — like the outside of a glass of iced water. AC refrigerant lines and chilled-water pipes are much colder than the air around them. In Thailand's hot, humid air the dew point is high, so an uninsulated cold pipe will almost certainly drip. The principle is in What are dew point and condensation?

Common causes

What you see	Possible cause
---	---
Drips only at certain points, e.g. fittings, valves, elbows	That section was not insulated, or not completely
Drips along insulation joints	Joints not glued or taped, or the glue/tape has come loose
Drips at hangers or ties	Insulation crushed, or the hanger touches the cold pipe directly
Insulation torn, sun-damaged and brittle, or chewed by rodents	Damaged insulation lets vapour reach the pipe
The whole run drips although the insulation looks intact	Insulation may be too thin for the space's temperature and humidity, or already wet inside
Glass-wool insulation on a cold pipe is soaked	The vapour-retarding jacket is not continuous; vapour has entered and condensed inside

Once water is inside the insulation, its ability to resist heat drops, the surface gets colder and more water condenses — a cycle. Insulation that is wet inside usually needs replacing, not just covering.

How to fix it

Find the cause first — note where it drips: one spot or the whole run (use the table above).
Uninsulated or damaged sections — insulate or replace with a type and size that suits the pipe, e.g. closed-cell elastomeric tube (AEROFLEX, Armaflex Class 1, MAXFLEX FR).
Joints — glue slit seams and butt joints as that insulation's maker instructs; for foil-faced insulation, seal joints with a suitable tape (aluminium foil tape). See Sealing insulation joints.
Hangers — use supports that do not crush the insulation, following the maker's recommendations.
If the whole run still drips — have the designer or a technician check whether the thickness suits the site. The thickness needed is calculated from pipe temperature, air temperature and humidity, pipe size and the insulation's thermal conductivity. No single number fits every pipe.
Outdoor pipes — protect the insulation from sunlight as the maker specifies.

When to call a technician

The water is coming from the AC unit, drain pan or a blocked condensate drain, not the pipe surface
Insulation has to be removed from refrigerant or chilled-water lines in a running system
Pipes are high up, near electrical wiring, or a large area of ceiling must be opened
Water is already dripping onto the ceiling or electrical equipment

These jobs should be done by an experienced AC technician or contractor.

Stop it happening again

Choose insulation and thickness for the real conditions from the start: How to choose pipe insulation
Keep the vapour retarder continuous through the whole system: What is a vapour barrier?
Inspect insulation periodically and repair tears or loose joints straight away

Sources

ASHRAE Handbook — Fundamentals, chapter Insulation for Mechanical Systems (condensation control on cold piping)
National Insulation Association, Insulation Outlook — Avoiding Condensation on Systems that Operate at Below-Ambient Temperatures
Armacell — technical information on condensation control and the ArmaFlex application manual
NOAA National Weather Service — definitions of dew point and relative humidity