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Bathrooms are hot. When the radiators are off all over the house, there is a good chance that the towel rail in the bathroom is still on. Bathrooms are also very wet, it’s what they are for! All of these factors are a recipe for condensation issues.

Add heat and water together and you get humidity. The bathroom is absolutely choc a bloc with humidity!

Then add to this heat and humidity a very cold surface such as your cold water toilet cistern or the cold water pipes which feed your cistern, the bath and the basin.

Your cold water, coming from the mains, is usually between 5 and 20 degrees celcius. Warmer if it comes from a cold water tank in the loft, but nevertheless, much much colder than the temperature in your bathroom.

Hole in floor caused by cistern condensation

large hole in floor caused by condensation running off of toilet cistern

This cold water makes the surface of the toilet cistern very cold and, or course, the surface of the cold water taps very cold too. The moment the humid hot ar from the bathroom hits the cold surface of the cold pipes or toilet cistern, it immediately turns to liquid, spilling water all over the floor. This is called sweating in the trade and you can be sure that in one week alone the cistern and cold pipes sweat quite a bit!

How about this image above to show you how much damage can be caused by a sweating cistern and imagine your shock if you were the one sitting on the loo when the floor gave way!

There are one or two ways to stop, or very much reduce, condensation on the toilet cistern. The first is to have plenty of air changes in the room. This is easy if your bathroom has a window; you just open it 2 or 3 times a day.

If you do not have a window however, you are reliant on the extractor fan to take the hot air (and the smells) out of the bathroom and new air to enter through the door. The new air however, is just old air from downstairs. Just as warm and just as likely to condense!

Bathroom extractor fan

Bathroom extractor fan for removing warm and humid air

We could add some hot water to the cistern. That would warm the cistern walls up and reduce condensation – BUT – the plumbing involved is beyond most DIYers and, if the loo is used frequently, it can be pretty expensive. The warm water also cools down very quickly when its mixed with the cold in the cistern, so not much of a solution really.

We could box the pipes and the cistern in and make sure that inside the boxing is a lot of insulation keeping everything nice and warm so the moisture in the air does not condense. This is a great solution where practical but once again, pretty expensive.

Boxing in pipework

Boxing in pipework to reduce condensation

So what can we do? DIY Doctor have thought about this problem for years. Our solution is about as practical as it gets. If we cant warm the water up, or change the air temperature, all we have left is to change the temperature of the cistern walls. So why don’t we insulate the cistern.

This, it turns out, is easier than you would imagine with the use of a simple Yoga mat and a tube of Everbuild’s Forever White Sealant.

Simply cut the Yoga mat to the shape of the inside of the cistern, making holes for the flush pipe and water inlet pipe, stick it to the surface and let it all dry.

Toilet cistern with Yoga mat glued to walls

Toilet cistern with Yoga mat glued to the inside to insulate the cistern walls from warm air

When the water is turned on, the yoga mat insulates the cistern but even better than that, some of the water squeezes up between the mat and the sides of the cistern. This behaves then exactly as a wet suit does to a diver. The thin layer of water gets warm and warms the contents of the cistern. Condensation is reduced hugely and your bathroom floor is saved!

Read about how to complete this simple project on this project page and watch our video too.


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