Summer is the season for backyard barbecues, days at the beach and family vacations. Unfortunately, your toilet sweating is another sign of the season. In fact, every summer millions of homeowners have to deal with toilets that perspire puddles of water onto the floor.
This problem is much more than just a mild annoyance: Condensation running down the toilet can seep under the flooring, rot the plywood subfloor and soak into the floor joists. It can also stain baseboard molding, turn drywall soggy and discolor wall paint with mildew. Read more to learn what causes a toilet to sweat and how fix it without having to upgrade your toilet.
Why Does My Toilet Tank Have Condensation?
When the weather turns hot and humid, there’s a lot of moisture in the air. At the same time the water entering the toilet tank is comparatively cold—about 50° to 60°F. When the warm, moist air hits the cool porcelain toilet surfaces, the air condenses, turns to water and soon drips onto the floor.
Although a toilet sweats only on warm, humid days, it can drop a surprisingly large amount of water in a very short time.
Several manufacturers make toilet-tank insulators they claim cure sweaty toilets, but most don’t work very well.
On a recent This Old House project visit, homeowner Ajay discovered just how damaging toilet condensation can be. After noticing water stains on the ceiling below his bathroom, he pulled down panels to investigate and found the culprit wasn’t a plumbing leak at all — it was condensation dripping from the toilet tank and surrounding fixtures that had ruined his ceiling. It’s a reminder that even though sweating seems minor, the accumulated moisture can cause real structural and cosmetic damage over time.
Check for a Running Toilet First: Before tackling condensation solutions, Richard Trethewey advises checking whether your toilet is running continuously: “The first thing I always look at with something like this is, is the toilet tank running? Because if we brought new water in all the time, it would make any toilet tank condense.” A constantly running fill valve keeps replacing tank water with fresh cold water from the supply line, dramatically worsening the sweating problem.
Pro Tip: Richard Trethewey, This Old House plumbing and heating expert, explains the phenomenon this way: “The cold water that comes in from the street will come into the house at 45 to 50 degree water temperature and that’s the same temperature water that gets into that tank. Now, out here on a summer day, you could be 75 or 80 degree air, right? And filled with humidity. Now, as that warm moist air approaches this colder surface, the temperature of the air drops below the dew point, the temperature of which moisture can remain in the air, and it condenses on the outside.”
Two Ways To Stop Your Toilet From Sweating
There are only two surefire ways to stop your toilet sweating:
1. Dry Out the Air in the Bathroom
Use an air conditioner or dehumidifier to dry out the air in the bathroom. However, this approach won’t work if you don’t have one of these units or don’t want to run it all summer.
2. Install an Anti-Sweat Valve
Install an antisweat valve in the water-supply line leading to the toilet. An anti-sweat valve adds a little hot water to the toilet water line, which raises the water temperature in the toilet enough to warm up the tank and bowl. That’s all it takes to keep condensation from forming, even in the most sultry weather.
Selecting Your Valve
Anti-sweat valves are sold at home centers and plumbing-supply dealers in both adjustable and preset types. When selecting your valve, pay the extra $10 or so for an adjustable model. It allows you to regulate the water temperature and shut down the hot-water side completely when it’s not needed.
For our installation, we chose the Adjusto-Temp adjustable toilet valve, from Universal Rundle (Model 5025-1, $33). This valve can be installed in the bathroom, behind a wall or below the floor (if there’s a crawl space or basement). Here, we show cutting the valve into a water-supply line in the basement directly beneath the toilet.
When choosing where to install the valve, keep in mind that orientation matters. As Richard Trethewey explained during a This Old House project install, he positioned the anti-sweat valve in the vertical position because “it has a couple of check valves that only work in the vertical position.” Those check valves are critical — without them, hot water could cross over into cold water lines, potentially delivering hot water at a cold water faucet elsewhere in the house.
Pro Tip: Richard Trethewey, This Old House plumbing and heating expert, notes that an adjustable anti-sweat valve is essential for seasonal control: “On a really humid day, you could have more mix and then turn it right off in the winter when you don’t need it.” This flexibility means you’re only using hot water when condensation is actually a problem, saving energy year-round.
How Do You Install an Anti-Sweat Valve?
There are three stages for anti-sweat valve installation: preparation, valve installation, and the temperature check.
Prepare
- Begin by shutting off the main water valve to the entire house.
- Then drain the hot-and cold-water lines by opening up all the sink and tub faucets and flushing all the toilets. To avoid scorching the internal components of the valve with a soldering torch, make all connections to the valve with brass compression adapters ($1.50 each).
- Loosely thread a 5/8 x 5/8-in. compression adapter into each of the three valve ports. Hold the valve against the horizontal cold-water pipe with the lower inlet port even with the pipe.
- Note where the center outlet port of the valve intersects the vertical pipe section; mark that spot on the pipe. Also mark the location of the lower inlet port on the horizontal pipe.
- Then use a hacksaw to cut out the pipe section.
- Next, solder a 90-degree L-fitting to the vertical pipe coming down from the toilet, then extend it with a 6-in.-long stub of 1/2-in.-dia. pipe.
- Use lead-free solder and a MAPP gas torch. Lead-free solder is relatively hard, and MAPP gas burns hotter and works more quickly than propane does.
- With the cold-water line ready for the valve, locate a nearby hot-water line and cut out a section to accept a new copper T-fitting.
- Splice the T-fitting into the line and assemble a short vertical riser pipe with a 90-degree L-fitting and short horizontal pipe stub.
- Point the stub toward the spot beneath the toilet where the valve will be installed.
- Solder the T- and L-connections.
- Add a length of pipe to reach the valve location.
Install the Valve
- Lightly coat the male threads of the three brass compression adapters with a pipe joint compound (pipe dope).
- Thread the adapters into the valve ports and tighten them with a wrench.
- Slide a nut and compression ring onto each pipe end and insert the pipes into the adapters.
- Coat each compression ring with pipe dope and thread the nuts onto the adapters.
- Tighten each nut with a wrench.
- Turn the main water valve back on and look for leaks.
Do a Temperature Check
- A slotted screwdriver is all you need to adjust the temperature of the water flowing through the valve.
- Start by turning the cold-water adjustment screw counterclockwise all the way.
- Then turn the hot-water screw clockwise until it bottoms out. That will fully open the cold-water side and shut down the hot water.
- Next, flush the toilet and open the hot-water side by turning the adjustment screw counterclockwise half a revolution.
- After waiting an hour or so, check for any condensation on the toilet. If necessary, open up the hot-water side of the valve a little more.
