Cold systems
Why do my chilled water pipes sweat, and how do I stop it?
Quick answer
Your chilled water pipe is colder than the air's dew point, so the moisture in the air condenses on it — the same reason a cold drink sweats in July. In DFW, summer dew points run in the 70s°F, and a 45°F pipe sits far below that, so it sweats all season long. The fix is closed-cell insulation thick enough to keep the outer surface above the dew point, wrapped in a vapor barrier with no breaks in it. On cold lines, thickness is sized by condensation control, not energy savings.
The physics, in plain words
Air carries water as vapor, and how much it can carry depends on temperature. Cool the air and at some point it cannot hold what it is carrying — that temperature is the dew point, and any surface sitting below it pulls the water out of the air as liquid. Your chilled water line runs around 45°F. In a DFW summer, the dew point sits in the 70s°F for weeks at a stretch. That is a 25 to 30 degree gap, and the pipe sweats hard the entire season.
Insulation fixes it by moving the cold surface. Put enough insulation on the pipe and the outside face of the covering sits near room temperature — above the dew point — and there is nothing cold left for the air to condense on. But that only works if water vapor cannot get through the insulation to the cold steel underneath. That is the part most sweating-pipe problems get wrong.
What a sweating line actually damages
The obvious cost is the water itself: stained ceiling tiles, drips on product and equipment, wet floors under the run. Above a finished ceiling or in a plenum, that same water feeds mold on the tiles, the deck and the insulation, and nobody sees it until it smells.
The expensive cost is under the covering. Water held against steel corrodes it — corrosion under insulation eats the pipe from the outside where nobody is looking, and on a chilled line the metal never gets warm enough to dry itself out. The insulation that was supposed to protect the pipe becomes the thing holding water against it.
And wet insulation stops insulating. Fiberglass that takes on water loses most of its R-value, the cold moves closer to the room air, and the sweating gets worse — a wet cold line is a problem that compounds, not one that stabilizes.
The vapor barrier is the whole game
Hot insulation forgives a flaw — a gap on a steam line loses some heat and that is the end of it. Cold insulation does not forgive. The vapor drive on a chilled line points inward, from the humid room toward the cold pipe, 24 hours a day, and it takes one tear, one unsealed seam or one unfinished termination to let the moisture in. Once inside, it condenses on the pipe and wicks down the run, soaking insulation far from the original hole.
That is why cold work fails from the outside in. The material matters, but the jacket, the seam sealing and the vapor stops at every termination, hanger and valve are what decide whether the system is still dry in five years. It is also why we treat cold and chilled line insulation as its own trade, not hot-pipe work at a different temperature — every seam glued or taped full length, every termination vapor-stopped, no shortcuts at the fittings.
The right materials: closed cell or nothing
For chilled water, the workhorse is elastomeric foam — flexible closed-cell rubber. The closed cells are their own vapor retarder, so when the seams and butt joints are glued full length, the run is sealed as one continuous barrier without depending on a separate wrap.
Where the stakes are higher — low-temperature process lines, outdoor runs, anywhere the insulation absolutely cannot take on water — we run cellular glass. It is closed-cell glass, absorbs no water at all, and is rated from −450 to 900°F. It costs more, and on cold service it is usually worth it, because it cannot fail the way a fibrous material fails.
Fiberglass on a cold line only works behind a flawless vapor jacket, and "flawless" is a lot to ask of a jacket that gets bumped, leaned on and re-hung for decades. Wet fiberglass on chilled service never dries. We would rather spec the material that does not care.
Thickness — and what to do if it is already wet
On hot lines, thickness is an energy calculation. On cold lines it is set by condensation control: enough inches that the surface of the insulation stays above the dew point on the most humid day your building sees, with margin. A colder fluid or a more humid space means more thickness, whatever the fuel math says. The energy savings on a 45°F line are real but small; keeping the system dry is the whole job.
If your line is sweating today through existing insulation, that insulation is most likely already wet — and on cold service it will not dry out on its own. The fix is to strip it and start dry: removal and re-insulation is its own service, and half the work is finding how far the water traveled. On the walk-down we shoot the run with an IR gun — wet insulation shows up cold and obvious. The survey is free, the written estimate takes 24 hours, and the number is (214) 603-9142.