Stop a toilet tank from sweating

Why a toilet tank sweats

Every time you flush, the tank refills with water straight from the cold supply line. In summer, or anywhere the incoming water is very cold, the tank walls get colder than the dew point of the air in the room. Moisture in the air condenses on the porcelain and runs down to the floor.

Sweating is worst in humid weather, after showers, and in bathrooms without a working exhaust fan. A toilet that runs or refills on its own makes it worse, because it keeps pulling fresh cold water into the tank. Leaks look similar but come from a specific spot, like the supply connection, the tank bolts or the base, and they drip whether the air is humid or not.

What makes it worse

Let's fix it

1. Make sure it's sweat, not a leakDry the whole tank, the supply line and the floor. Wrap a dry paper towel around the supply connection and the tank bolts underneath. If beads of water form evenly all over the outside of the tank, it's condensation. If water shows up at one spot, it's a leak; tighten or replace that part, or there's a separate guide for a toilet leaking at the base. Add a few drops of food coloring to the tank: colored water outside the tank means a crack or leaking bolt, and color in the bowl without flushing means a leaking flapper.
   Done

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2. Stop the toilet from runningA toilet that runs or refills on its own keeps filling the tank with cold water, so it never warms up. Listen for a hiss or watch the bowl for a trickle. If it's running, fix that first; the guide for a running toilet walks through the flapper and fill valve.
   Done

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3. Lower the bathroom humidityRun the exhaust fan during every shower and for at least 20 minutes after. Keep the door open when you can, and use a dehumidifier in a damp bathroom or basement. A hygrometer shows the humidity; aiming for roughly 30 to 50 percent indoors keeps condensation down. If the fan doesn't pull air, fix it first; the bathroom exhaust fan guide covers that.
   Done

   Something went wrong?

   • The fan doesn't clear the steam Hold a square of toilet paper to the grille; if it doesn't stick, the fan or duct needs work.

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4. Insulate the inside of the tankShut off the water, flush, and sponge the tank dry. Cut the foam panels from a tank liner kit to fit the front, back and sides, keeping them clear of the flapper, chain, fill valve and flush handle. Stick them on with the kit's adhesive and let it cure for the time on the package before turning the water back on. The foam keeps the cold water away from the porcelain so the outside stays closer to room temperature.
   Done

   Something went wrong?

   • The foam won't stick Dry the tank completely and clean it first; damp porcelain won't hold adhesive.

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5. Add warm water with a mixing valveIf it still sweats, an anti-sweat mixing valve tees into the hot water line and blends a little warm water into the toilet's supply, so the tank fills with water that isn't ice cold. It needs a hot water line near the toilet and some soldering or push-fit work, so many people have a plumber install it.
   Done

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Watch it done

How To Fix A Sweaty ToiletBy Ace Hardware on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it's a leak instead

If the water comes from the base, not the tank, we have a guide for a toilet leaking at the base.

If the windows sweat too, the house may be humid; see the guide for condensation on windows.

Toilet sweating questions