Why is my toilet sweating?

Find out if it’s condensation or a leak, then pick the fix that matches your bathroom.

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Quick answer

Your toilet sweats when cold water in the tank chills the porcelain below the dew point of the bathroom air. Run the fan and cut humidity first. If it still drips, line the tank or add an anti-sweat valve.

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The checker compares two numbers. The dew point is the temperature at which the air in your bathroom can’t hold its water vapor any more. If the outside of the tank is at or below it, water beads on the porcelain, just like on a glass of iced tea.

The tank’s outside is close to the temperature of the water inside, at least right after a flush. That’s why sweating is worst when the bathroom is humid and the tank refills with very cold water, as it can in homes on wells.

The table shows why a shower can set off a tank that stays dry the rest of the day. At 72°F and 40% humidity, the dew point is about 46°F, colder than most tank water. Push the humidity to 70% at 75°F and the dew point rises to about 65°F, warmer than a lot of tap water.

EPA’s moisture guide says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent. Holding the bathroom in that range costs the least, and it helps the walls and ceiling too.

Dry the tank, the bolts and the supply line with a towel, then watch for ten minutes with nobody using the toilet. Condensation comes back as fine beads across the cold part of the tank. A leak comes back in one spot and drips from there.

A running toilet makes sweating worse, because it keeps sending fresh cold water into the tank. Fix the flapper or fill valve first, then see whether the sweating is still a problem.

Start with the air. EPA’s guide says to run the bathroom fan or open the window when showering, and to use air conditioners or dehumidifiers when needed. If the fan runs only while the light is on, a timer keeps it going after the shower; see bathroom fan timer.

If the bathroom is already dry and the tank still drips, the problem is the water, not the air. That’s when a liner or a valve earns its place.

A liner works the same way EPA describes for pipes: cover the cold surface with insulation. Foam on the inside walls slows the cold water from chilling the porcelain, so the outside stays warmer than the water. Most kits are self-adhesive foam sheets you cut to size.

I haven’t linked a liner here. The ones I checked on Amazon were store brands with no maker documents, or a multipack whose listing didn’t match the maker’s own description, so I can’t vouch for what would arrive. If you buy one, look for a named maker that publishes the material and size.

The catch is room. Tanks are tight around the fill valve, the overflow tube and the flapper chain, so cut panels to clear every moving part. A panel that lifts off and floats can jam the flapper, which turns a sweating toilet into a running one.

An anti-sweat valve tees a little hot water into the toilet’s cold supply. Legend’s T-20 is factory set to 30% hot and 70% cold; the T-20A in the kit lets you turn the hot share down from 30% to nothing with an adjustment screw. Compression nuts and ferrules are available, per Legend’s sheet.

The math is simple. With 55°F cold water and 120°F hot, a 30% mix fills the tank at about 75°F, well above the dew point in most bathrooms. At 10% hot, it’s about 62°F. Set the T-20A to the lowest share that keeps your tank dry.

The trade-off is hot water. At 30% hot, a 1.28 gallon flush draws about 0.38 gallons from the water heater. That’s why the adjustable T-20A makes sense: turn it down in winter if the tank stops sweating, or to zero when you don’t need it.

You need a hot line near the toilet, which usually means a sink on the same wall or a line below the floor. If you don’t solder copper or have never worked on supply lines, this is a job for a plumber. A thermostatic mixing valve is a different part, made to limit hot water at fixtures, not to warm a toilet tank.

Best for measuring humidity

Govee H5075

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Best for clearing shower steam

Lutron MA-T51MN timer

$59.90 at Amazon

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Price as of Oct 24, 2026, 5:30 AM EDT

Best for cold well or city water

Legend T-20A (111-154)

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Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.

The Govee is in the kit because the fix depends on two numbers you can’t feel: humidity and dew point. Govee lists its accuracy at ±0.5°F and ±3% relative humidity, with a 2-second refresh, and the listing says it keeps max/min records, so you can see what a shower does to the room.

The Lutron timer keeps the fan running after the shower without anyone remembering to turn it off. Lutron’s spec sheet lists a tap-on countdown of 5 to 60 minutes, a one-minute warning, a 3 A fan rating and a neutral connection requirement. If your switch box has no neutral wire, or you don’t do electrical work, have an electrician fit it.

1. 1
   Set the hygrometer near the toilet

   Away from the shower spray and the heat vent.

2. 2
   Read it after a shower

   The highest humidity is what makes the tank sweat.

3. 3
   Read it again an hour later

   This shows whether the fan is clearing the air.

4. 4
   Measure the cold water

   Run the cold tap a minute, then put a thermometer in a glass.

5. 5
   Measure the hot water

   Same way at the nearest faucet.

6. 6
   Run the checker

   Use the worst readings, not the best.

If the humidity drops back under 50% within an hour of a shower and the tank only sweats for that hour, the fan is doing its job and a towel or a short wait may be all you need. If the humidity sits above 60% all day, fix the air first; neither a liner nor a valve will keep the walls and mirror dry.

The bowl can sweat too, mostly right after a flush, because it refills with the same cold water. A liner can’t reach the bowl, but an anti-sweat valve warms the water that refills both the tank and the bowl, and lower humidity helps everywhere.

If only the bowl drips and only after flushes, wait a few minutes after flushing and check again; once the water in the bowl warms toward room temperature, it rises above the dew point and the beads stop forming.

Your cold water temperature isn’t fixed; it changes with the seasons, so a tank that stayed dry one month can sweat the next. Measure the water again when the sweating starts, and run the checker with that number before you decide a liner is enough.

If the toilet is old, runs, or the floor around it is already soft, replacing it can make more sense than lining the tank. See best toilets and cost to replace a toilet. The same cold-surface rule applies to exposed cold pipes in the bathroom; pipe insulation stops them dripping too.

Sources

• Legend Valve, Anti-Sweat Toilet Tank Valves Model T-20 / T-20A submittal sheet, legendvalve.com, read Oct 24, 2026.
• Lutron, Maestro Timer Switch MA-T51MN specification submittal 3691243a, assets.lutron.com.
• U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, epa.gov, read Oct 24, 2026.
• Govee, H5075 Bluetooth hygrometer thermometer product page and FAQ, us.govee.com, read Oct 24, 2026.
• Alduchov, O. A., and Eskridge, R. E. (1996), Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology 35: 601–609.
• Amazon product data for B002KU953G, B0017PNRTS and B07Y36FWTT, read through the Creators API on Oct 24, 2026.