Condensation on toilet cisterns: How to stop 'toilet sweating' and why it happens

white toilet cistern
(Image credit: Getty)

The presence of condensation on toilet cisterns – a pesky phenomenon also known as 'toilet sweating' – is something that the majority of homeowners will, at some point, have encountered. 

Although a cistern covered in running drops of water might look rather unsightly and require a wipe down to avoid water pooling around the toilet seat, it is not an issue that points to any serious problems. That said, it is always useful to understand how to stop condensation in your bathroom.

If you are set on ridding yourself of toilet sweating, there are steps you can take to minimise this kind of condensation — our guide takes you through them. 

What causes condensation on toilet cisterns?

Just as with window condensation, when warm air hits a cold surface, moisture is pulled (or ‘condensed’) out of the air, settling on the cold surface as drops of water. 

“We cannot stop condensation,” says energy efficiency expert Tim Pullen. “In a WC, the cistern becomes a very cold surface every time the toilet is flushed and refilled with cold water. If the room is warm, condensation will then form on the cistern."

Obviously the warmer the room, the more condensation will form on any cold surface.

Should I be worried about toilet condensation?

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While it might be annoying, toilet cistern condensation is actually to be expected. That said, if left unchecked, it could lead to other, more serious problems.

If you were to allow water to constantly run down the cistern to the floor, damp could eventually form, leading to mould in bathrooms. If the water running down from the toilet was to penetrate your bathroom flooring, it could even lead to rotting issues with timber floor joists too. 

Providing you keep on top of the condensation by wiping the cistern down to dry it off, you should not run into any big problems. 

“In extreme cases, where condensation is forming on the walls and/or the ceiling, then extra insulation would be needed,” says Tim Pullen. “Keeping the walls and ceiling warm will stop the condensation forming on them — a product such as Sempatap is specifically designed for this purpose.”

How to stop toilet condensation

There are a couple of steps you can take to prevent – or at least reduce – toilet sweating. 

“Maintaining a cooler room by fitting a programmable radiator valve to keep the room a little cooler than the rest of the house would probably sort the problem out,” says Tim. “Or, if you don’t have one already, install a good extractor fan. This is more to do with maintaining good air flow through the room though.” It is also important to make sure the room gets plenty of fresh air — simply opening the windows once a day will make a notable difference.

The best bathroom extractor fans will ensure moisture is kept under control in the bathroom and should prevent toilet condensation becoming an issue. 

If the problem persists, there are other routes you can take, including insulating the cistern or fitting an anti sweat valve like this one on Amazon for £36.63 (which may need separate UK bolts)

white bathroom extractor fan set in wall with bright green tiles

A decent extractor fan can help the problem but increasing the ventilation in your bathroom (Image credit: Getty)

How do you create an insulated toilet cistern?

Insulating your toilet cistern might sound a little odd but it could just ensure your toilet sweating becomes a thing of the past.

There are DIY hacks you might like to consider, such as using bubble wrap, to line the inside of your cistern, but using rolls of closed cell foam material or waterproof insulation is also an idea worth mulling over.

In all honesty, here in the UK, insulating the interior of a toilet cistern is not a common job, and unless you are seriously concerned about excessive condensation forming on your toilet tank it could be unnecessary providing you incorporate good ventilation. Emptying the cistern, cutting the insulation to fit and glueing it into place is time consuming and fiddly. 

inside of an insulated toilet cistern

Lining the inside of your cistern with insulation should put a stop to toilet sweating — but it is a fiddly job.  (Image credit: Getty)

What is an anti sweat valve?

Again, this is not a common solution in the UK, although it is possible to purchase anti sweat valves online.

These are basically small plumbing parts that are fitted into the cistern to introduce a small amount of warm water into the cold water that feeds into the toilet tank. Depending on your DIY plumbing skills, you may need to call in a professional to fit the valve. 

Tips to reduce toilet condensation

Aside from those mentioned already, there are a few other methods you can use if you are suffering from toilet sweating. These include:

  • Wait to flush the toilet until after showering or bathing — the water in the cistern should be a little less cold if it has been sitting in there a while, thus reducing condensation
  • Open windows regularly for good ventilation
  • Install a condensation drip tray. These plastic trays fit under the cistern and catch excess moisture — they don't look terribly attractive but they stop water pooling on the floor
  • Invest in a dehumidifier to remove excess moisture from the air
Natasha Brinsmead

Natasha is Homebuilding & Renovating’s Associate Content Editor and has been a member of the team for over two decades. An experienced journalist and renovation expert, she has written for a number of homes titles. Over the years Natasha has renovated and carried out a side extension to a Victorian terrace. She is currently living in the rural Edwardian cottage she renovated and extended on a largely DIY basis, living on site for the duration of the project. She is now looking for her next project — something which is proving far harder than she thought it would be.