A pipe under the sink sweats when the cold supply line or the trap runs colder than the cabinet air's dew point, so water condenses on the outside of the metal — the same mechanism as a sweating toilet tank, made worse by the sealed cabinet trapping humid air around it. Dry the pipe and watch it: an even film that returns within an hour is condensation; a wet spot at one fitting that keeps reappearing is a leak. A foam pipe sleeve fixes condensation; a leak needs a wrench.
Why the pipe under a sink sweats
The cold-water supply line reaching the sink, and the trap just below it once cold rinse water has run through, both sit close to the temperature of the water inside them — usually well under 60 °F. That is the identical mechanism covered on sweating pipes and on a sweating toilet tank: once the air around a cold surface is humid enough that its dew point sits above the surface temperature, water condenses directly onto the metal, no leak required.
What changes under a sink is the air the pipe sits in. A cabinet door closes off nearly all of that air from the rest of the kitchen or bathroom, so anything that adds moisture nearby — a dishwasher venting steam, hot rinse water going down the drain, or simply a humid room — raises the humidity right around the pipe and has almost nowhere to go once it is in there.
Why a sealed cabinet makes it worse than an open pipe
A pipe running through open space in a basement condenses only when the whole room is humid enough. A pipe in a sink cabinet condenses more easily than that, because the cabinet is small enough that ordinary daily use — steam from the dishwasher, warm air trapped after washing dishes by hand — can push the air inside it past the dew point even on a day when the rest of the house reads comfortably dry.
That is the same trapped-air effect covered on mold under the kitchen sink, just one step earlier: humidity condensing on the pipe is often the first sign of the exact conditions that page covers once mold has already taken hold.
Condensation or a leak? Read the pipe
| What you find | Likely cause | What to do |
|---|---|---|
| Fine, even film across the whole pipe or trap | Condensation | Dry it, insulate with a foam sleeve — see below |
| Wet spot at one fitting only, dry everywhere else on the pipe | Slow leak at that joint | Tighten or replace the fitting; do not just insulate over it |
| Cabinet floor damp only after the sink or dishwasher runs | Drain-line or trap leak tied to use | Check the trap connection and drain gasket |
| Cabinet floor wet with nothing run in hours | Supply-line leak under constant pressure | Shut off the valve, repair before it soaks the floor further |
| Mineral crust or white residue around a fitting | Leak that has been dripping for weeks | Treat as active plumbing failure, not condensation |
The dry-and-watch test
Clear the cabinet, wipe the supply line, the trap and every fitting completely dry with a towel, and leave the door open for an hour. Condensation returns as a fine, even film across the whole cold surface, front and back, on a humid day. A leak reappears at the same single point — usually a compression fitting, the trap slip nut, or where a supply line threads into the shutoff valve — regardless of how humid the room is.
If you are not sure which fitting to watch, wrap a strip of dry paper towel around each one and run the sink, then the dishwasher, checking the strips after each. A strip that stays dry through both while the pipe still sweats is generally condensation; one that wets during a specific fixture's use points straight at that fixture's connection.
Fixing condensation: insulate the cold line and the trap
Closed-cell foam pipe insulation, sold as pre-slit tube sleeves at any hardware store, slips over the supply line and the trap and keeps the water inside from chilling the outer surface below the cabinet air's dew point. Tape or glue every seam and joint — a gap in the foam is where sweating starts again, the same way a missed seam undoes a foam liner in a sweating toilet tank.
It is inexpensive, takes about fifteen minutes for a full sink cabinet, and needs no plumbing skill beyond cutting the foam to length and working it around the trap's bends.
When it is the cabinet air, not just the pipe
If the pipe keeps sweating right through a fresh foam sleeve, or if the cabinet floor, the underside of the countertop and the pipe are all damp at once, the cabinet air itself is running too humid for insulation alone to keep ahead of. Check your kitchen's or bathroom's reading against the dew point calculator to see how close the room runs to condensing on cold surfaces in general, then add airflow to the cabinet — a vented door insert, or simply leaving the door cracked overnight — the same fix mold under the kitchen sink recommends once trapped humidity has gone further than a wet pipe.
Insulating foam pipe sleeves and raising surface temperatures elsewhere in the room work the same way; see insulation and heating fixes for options beyond the cabinet itself.
When standing water under the sink becomes a mold problem
A pipe that sweats for an afternoon and gets wiped dry rarely causes lasting damage. One that sweats daily and drips onto the cabinet floor is a different matter — sealed particleboard swells and delaminates within days of standing water, and the dark patches that follow are exactly the mold covered on mold under the kitchen sink. Treat a pipe that keeps re-wetting the same spot as the early stage of that problem, not a separate one.
Protecting the cabinet floor while you fix it
Wipe the pipe and the floor dry daily until the foam sleeve is on and holding, and keep cardboard boxes or paper grocery sacks off the cabinet floor in the meantime — both soak up standing moisture and mold before the wood underneath even stains. A rubber or plastic tray under the trap catches any drip that returns while you watch for it, and it is a two-second check the next time the cabinet is open.
Common questions
Is condensation on a pipe under the sink the same as a leak?
No. Condensation coats the whole cold surface of the pipe or trap evenly and returns as a fine film within an hour of drying it. A leak wets one fitting and keeps reappearing at that same spot, whether or not the room is humid.
Do I need to insulate the trap as well as the supply line?
Yes if the trap is cold to the touch, which it usually is right after the sink drains. Foam sleeves are sold pre-slit for straight runs and can be worked around the trap's bend; leaving the trap bare while insulating only the supply line just moves the sweating to the part you skipped.
Why does the pipe under my sink only sweat after the dishwasher runs?
A dishwasher vents hot, humid air partway through and at the end of its cycle, and some of that moisture reaches the sink cabinet through the drain-line cutout. It raises the cabinet's humidity just long enough to push a cold pipe past its dew point, then settles back down once the cycle finishes.
How long can pipe condensation drip before it damages the cabinet floor?
Standing water can start softening sealed particleboard or plywood within days, faster on unsealed or bare wood. A cabinet floor that feels soft, spongy or discolored at the base of the pipe has usually been taking on water longer than the visible dampness suggests.
Will a foam pipe sleeve fix a trap that is already leaking?
No. Insulation only addresses condensation on a sound pipe — wrapping foam over a leaking fitting hides the drip without stopping it, and the trapped moisture underneath makes the cabinet worse, not better. Tighten or replace the fitting first, then insulate.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions
- ASHRAE Standard 160 — Criteria for Moisture-Control Design Analysis in Buildings — condensation risk assessed against interior surface temperature relative to the dew point
- US EPA — A Brief Guide to Mold, Moisture and Your Home — standing moisture against building materials, including cabinetry, as a condition that supports mold growth