Condensation on Toilet Cistern - How to Reduce and Minimise Condensation Buildup on Toilet Cisterns and Pipework

Summary: Condensation on toilet cistern and pipework in bathrooms and how to solve condensation issues. In this guide you will learn about the most likely causes of condensation on toilet cisterns and pipework and also provide some handy tips on how to drastically reduce condensation buildup in high moisture and humidity areas such as the bathroom.

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Condensation under toilet cisterns can cause unseen problems. The warm air in the bathroom hits the underside of the toilet cistern and cold water pipes and condenses immediately (for more about this see our project on condensation).

DIY Doctor has devised a method of minimising condensation on toilet cisterns and cold water pipes. This is a huge problem for some houses and can lead to bathroom carpets and floorboards actually rotting from the constant drip of water from the water pipes and cisterns in the toilet or bathroom.

Once the floorboards are wet is is only a matter of time before the joists themselves begin to suffer and if you find yourself in this position you might want to take a look at our project on replacing floorboards. We also have an excellent video on this page about replacing floorboards under a toilet.

Cistern condensation can cause rotted floorboards

Rotten bathroom floor under dripping cistern condensation

Problems With Cistern and Pipe Condensation in Modern Bathrooms

It seems to be a modern trend to put the toilet cistern out of site. This cistern can be hidden inside a false wall or have a purpose made boxing made around it to keep it out of sight.

While this makes the whole bathroom look much neater and tidier it can lead to huge problems if the toilet cistern and cold water pipes are not insulated. Condensation is far less dangerous when it can be seen and rectified but if it is happening out of sight, all sorts of damage can result.

How to Stop or Reduce Condensation on Toilet Cisterns and Water Pipes

Boxing in Pipes

Pipe boxing to keep cold water pipes warmer

Use boxing and insulation to warm cold water pipes

If the condensation problem exists in your bathroom or toilet, boxing in the pipes where ever possible can help. This might warm the water going into the cistern slightly allowing less condensation to form.Make sure you lag the pipes and/or place insulation inside the boxing also. You can find out more about pipe boxing in this project.

The hot humid air from the room condenses on the colder water pipes and underside of cistern so any insulation you can use will make things better. Boxing on it's own however can be dangerous as you can no longer see the condensation forming. Don't forget to insulate as well as box in the pipes.

Adding Hot Water to Your Toilet Cistern

There are fittings available in the US called anti-sweat valves which can add a little hot water to your cistern when the cold water enters after you have flushed. This can be expensive and if it is a loo which is not used often, can be a complete waste.

This anti-sweat valve can be achieved by connecting your hot water basin or bath supply to the water feed as it enters the cistern. The hot feed should be controlled by an in line service valve (such as that used on a washing machine hose) which will allow you to turn down the amount of hot water mixing with the cold as it enters the cistern.

Remember, the cistern only needs to be at the same temperature as the air outside to avoid condensation.

Warming the cistern water will reduce condensation

Anti-sweat valve for adding warm water to toilet cistern - Available form Amazon here

 The problem as mentioned above, is that, in any relatively small room, the air gets quite warm. Warm air carries moisture (as stated in our condensation project linked above) which will condense as soon as it hits a cooler surface. The toilet cistern, being full of cold water, is a cold surface as are the cold water pipes running to the cistern, the basin and the bath.

Service valve for adding hot water to cistern

Add hot water to toilet cistern via a service valve

Ventilate the Toilet or Bathroom

The universal answer to condensation in bathrooms, whether it is on the cistern, the pipes or the tiled walls,  is of course to ventilate the room to keep everything at a fairly equal temperature but the problem still exists in small bathrooms; therefore we need to make the surface of the cistern warmer and this can be done in a relatively simple way.

Extractor fan in bathroom

Condensation can be reduced by extracting the warm, humid air - Available here form Amazon

Extractor fans will not only remove foul smells from the bathroom but circulate the air at the same time. This draws the hot air up to the fan where it can escape without condensing.

It is important to ensure you have some fresh air entering the bathroom regularly, as with every other room in the house.

Regular air changes avoid condensation much more easily than any other method. Opening the windows at least once a day is effective insurance against condensation.

The DIY Doctor Method for Curing Condensation on Toilet Cisterns

Buy a yoga mat! They can often be bought at car boot sales and (after the onslaught of new years resolutions regarding losing weight, getting fitter etc) and websites like Ebay and Gumtree. Go for the thin, very dense kind.

Full details on our fix can be seen in the video below.

Firstly and most importantly, turn off the cold water via the isolation valve if you have one. If you do not, turn the water off at the mains and fit an isolation valve for ease in the future. There is an excellent project, complete with video, on fitting and isolation valve here.

Cold water isolation valve

Use isolation valves to turn water off to toilet cistern

You can strip out the contents of your cistern, or simply work around them. Flush the cistern to remove as much water as you can then, and using a sponge or rags etc, wipe out the cistern until it is completely dry. Then use a hairdryer to warm up the inside.

Use a yoga mat to counter cistern condensation

Toilet cistern condensation countered with a yoga mat

Flush the toilet cistern out as much as possible.

Removing flush unit from toilet cistern

Get to the cistern parts by removing the flush unit

Remove any removable parts, such as the flush unit, from the cistern. By the way it is not a very difficult job to remove the toilet completely, dismantle it and work in comparative ease. See the video on replacing a toilet in this project here to see how this can be done. Sponge out last of water and dry the inside of the cistern.

Removing flush pipe from toilet cistern

Flush pipes can be removed from toilet cisterns

Cut the yoga mat into sections which will fit on the inside of the cistern and stick them in place using a suitable waterproof glue or sealant. We advocate the use of a sealant called Forever White which is excellent for this purpose.

Lining a toilet cistern with a yoga mat

Cutting yoga mat to line toilet cistern

The inside of the cistern needs to be completely dry for this to work properly. Make sure none of the mat fouls any of the moving parts of the cistern.

Anti condensation method using bathroom sealant

Using sealant as anti condensation on toilet cistern

When your toilet cistern is lined it should look a little like this. Leave it for 8 to 10 hours for the sealant cure completely.

Lined toilet cistern safegiard against condensation

Safeguard against condensation by lining your toilet cistern with an old Yoga mat

You may need to reset your float valve if you have the older type of cistern (see our cisterns and tanks project) because the cistern will now hold a little less water.

The yoga mat acts as insulation on the inside of the tank which gives the outside an opportunity to warm up. The cold water within the cistern will find it's way between the walls of the cistern and the inside of the yoga mat. This thin film of water will warm up quickly and will keep the outside of the cistern warmer.

It is vital therefore, that you use a sealant which will definitely hold the yoga mat in place. There is just no point in using a cheap sealant at the yoga mat will come unstuck which will not only allow your cistern to start sweating condensation again, but it could foul the moving parts of the toilet.

Forever White Sealant

Excellent bathroom sealant in Forever White - Available from our online store here

Avoiding Condensation on Cold Water Pipes

Cold water pipes are, obviously, colder than the warm, humid air in the bathroom and as such are prone to just as much condensation as the cistern itself. This problem can be effectively reduced in a couple of ways other than the boxing in method we spoke about at the top of the page.

  • Method 1 is to lag the pipes in the bathroom just as you would if they were in the loft. There is not always the room for this underneath a toilet cistern but if there is the lagging can be disguised by painting it the same colour as your bathroom walls or skirting
  • 15mm pipe lagging

    Lag the bathroom pipes with 15mm pipe lagging

  • Method 2 takes up a lot less room but is not quite so effective. It is however, considerably more attractive! The tubes in the image are called pipe snaps and they are pre-curled strips of plastic which can be cut to length easily as shown in the video. They simply clip over the pipe and make it just a touch warmer and less prone to condensation
Warm and hide copper pipes using pipe snaps

White pipe snaps to hide and warm copper pipe

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