Why Is My Toilet Tank Sweating? Causes and How to Stop It

Before you treat it as sweat, make sure it is not a leak. A tank that sweats is wet all over its outer face, below the water line. A leak usually shows up in one spot, such as a tank bolt, the supply line nut or the base of the toilet.

Graphic: summary of this guide, made by our team from the cited sources.

Sweat or leak? Check where the water is

Dry the tank, the bowl, the supply line and the floor with a towel. Then check back after a while and see where the water returns.

What you see	Likely cause	What to do
Beads of water all over the tank, worse on humid days or after showers	Condensation (sweating)	Lower the humidity; see the fixes below
Tank sweats and the toilet refills on its own between flushes	A leak keeps adding cold water	Do the dye test and fix the flapper
Drips from one bolt or nut under the tank	Leaking tank bolt or supply connection	Tighten or replace the part, or call a plumber
Water at the base of the toilet with a dry tank above it	Not sweat: likely a seal at the floor	Call a plumber
Water level in the tank drops with the water off	Flapper or flush valve leak	Use the water level test

Sources: condensation causes from Michigan State University Extension; dye test from EPA WaterSense; water level test from Fluidmaster. The "where the water is" check is our method.

Why the tank sweats: 4 causes

1. Humid air in the bathroom

Hot showers fill a small room with moisture. MSU Extension explains that warmer air can hold more water vapor, and when that air cools against a surface colder than the air, the extra vapor condenses on it. A toilet tank full of cold water is that cold surface. The EPA's guide to mold, moisture and your home says condensation can be a sign of high humidity, and recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent.

The EPA says a humidity meter is a small, inexpensive instrument (it lists $10 to $50) sold at many hardware stores.

2. Cold water in the tank

Every flush refills the tank with water straight from the cold supply line. In summer the gap between that water and the room air is wider, which is why MSU Extension lists toilet tanks and cold water pipes as summer condensation problems. The cold water pipe to the toilet can sweat for the same reason.

3. A toilet that keeps refilling

A tank that leaks into the bowl refills on its own, so fresh cold water keeps coming in and the tank never has time to warm toward room temperature. If the toilet also runs every so often, our guide on why a toilet runs every few minutes and how to fix a phantom flush covers the repair.

Two quick tests:

1. Dye test. EPA WaterSense's leak-checking guide says to put a few drops of food coloring in the tank. If any color shows up in the bowl after 10 minutes, you have a leak. EPA WaterSense lists worn flappers as a common household leak and says the flapper should be checked periodically and replaced at least every five years.
2. Water level test. Fluidmaster's tank leak test has you turn off the water at the shutoff valve without flushing, mark the water level, and wait 30 minutes. The water drops to the level of the leak and stops. If it drops to the lip of the flapper, Fluidmaster says the flapper may be worn or the seat it rests on may be damaged. Our guide on why a toilet's water level drops by itself goes through each result.

4. Poor air movement

A small bathroom with the door shut and no fan running keeps the moist air in place after a shower. The EPA recommends running the bathroom fan or opening the window when showering.

How to stop a toilet tank from sweating

Start with the cheapest step and only move on if the tank still drips.

1. Run the fan and dry the air. Run the bathroom exhaust fan or open a window when you shower, as the EPA recommends. Opening doors to get air moving also helps prevent condensation, according to the same EPA guide. The EPA also says to use air conditioners or dehumidifiers when needed.
2. Fix any leak. If the dye test or the water level test shows a leak, replace the flapper or the worn part so the tank stops refilling.
3. Wipe up and protect the floor. American Standard says condensation can damage flooring if it is neglected for long periods, and the EPA says to act quickly to dry wet surfaces. Wipe the tank and floor and put down a towel until the fix is done.
4. Insulate the pipe. The EPA says to cover cold surfaces, such as cold water pipes, with insulation. MSU Extension suggests tubular or wrap insulation around water pipes to prevent condensation.
5. Insulate the tank from the inside. MSU Extension lists rigid waterproof insulation on the inside of the tank as one way to warm the tank surface. Liner kits go on the inside walls of an empty, dry tank. Follow the kit's instructions.
6. Add an anti-sweat valve. MSU Extension's other fix is a mixing valve on the cold supply line, which brings some hot water into the tank and can warm it enough to stop condensation. One example, the Legend Valve T-20 anti-sweat valve submittal sheet, lists a factory setting of 30 percent hot and 70 percent cold, with an adjustable model that turns down to all cold. The valve needs a hot water line nearby, so it usually means a plumber cutting into the supply pipes.
7. Consider a lined tank. The American Standard knowledge base answer on tank condensation says it offers lined tank toilets that are less likely to cause condensation. If you are replacing the toilet anyway, ask about a lined tank.

When to call a plumber

• The water is at the base of the toilet, not on the tank.
• A tank bolt or supply connection keeps dripping after you tighten it by hand.
• You want an anti-sweat valve, which means connecting to a hot water line.
• The floor around the toilet feels soft or you see mold that keeps coming back.