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Condensation of moisture on the outside of toilet tanks is more than a mere annoyance. It encourages mildew. Water that seeps onto the wall or drips from the tank to the floor may cause tiles to loosen or wood to rot.

Tanks sweat because cold water inside the tank cools the porcelain surface, so that moisture in the warm air of the room condenses on it. The housewife’s usual panacea, a cloth jacket that fits over the tank, quickly becomes saturated itself and does little to stop the dripping.

Unless the tank water is downright cold–below 50 degrees–condensation usually can be stopped by lining the tank with a waterproof insulating material such as foam rubber. Liners are available at some plumbing supply stores, or you can buy sheets of foam rubber 1/2 inch thick at a department store or upholstery shop. Cut it to size with scissors.

If your incoming water supply is often colder than 50 degrees, the only sure way to stop condensation is to raise the temperature of the tank water. This is done by installing a temperature valve, which mixes hot water with the cold water supplying the tank. A hot water line is generally nearby at a sink or tub.

To install insulation pads, first drain the tank and sponge it dry. Measure the inside width and depth of the tank, and the height from the bottom of the tank to a point 1 inch above the overflow pipe. Cut four pieces of 1/2 inch thick foam rubber to fit the front and back and each side.

Trim one 1 inch from the width of the front and back pieces so they will abut the side pieces. Make a cutout in the front piece for the flush-handle bracket, and make sure the pads do not interfere with any other moving parts. Apply a liberal coating of silicone glue or rubber cement to the tank surfaces and press the pads in place. Let the glue dry 24 hours before refilling the tank.

DIY Plumbing – How to Sweatproof a Toilet Tank