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I recently changed some of the pipes around under my shower, however now I have discovered this pipe under the floor which I have not interfered with at all that seems to have been sweating for some time. The ones I rerouted are perfectly fine, but this one seems to hold ground temperature water and the tee shown in the picture routes off to a toilet cistern. I haven't done anything to this pipe though I may have jostled it slightly to work on the others. It appears to have been sweating for some time (House is ten years old) and it was black along its length until I wiped it clean.

The water inside the pipe is at about 8 degrees c and the house is heated to about 18. The film of water shown in the picture formed overnight. What puzzles me is that the water does not appear to be coming out of the joint, rather it is forming both on the metal tee and on the plastic pipe. The tee is the lowest part of the pipe so the water could not be running up the plastic pipe, so it is almost certainly sweat not leaking, but I am open to being corrected. The wooden beam the pipe passes through shows signs of moisture at the bore.

Is this really sweat? Why so much? Is it normal? Is the pipe doing this throughout its whole length? Should I replace the tee? Should I just insulate this part of the pipe that I have access to?

Sweaty pipe

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  • You might check the humidity in that area and see if it's unusually high.
    – jwh20
    Feb 3, 2022 at 12:26
  • Quite common when cold water(in pipe/tank/glass) and warmer moist air meet. One reason they make insulation products for pipes/ toilet tanks/ and water glasses. The other pipe in the picture is a hot water pipe(warm water, dry pipe). To check, dry pipe and watch if any drips come out from the joints(that is a leak), or just form on the pipe.
    – crip659
    Feb 3, 2022 at 12:42
  • I'm confused, you say it's under your shower but I also see connector hoses in the photo. Is this in the bathroom space or in the floor void? Do you have hoses in enclosed space? How are you getting such a good picture? Is the area under your shower in an unfinished basement or such?
    – jay613
    Feb 3, 2022 at 16:03
  • That sounds perfectly "natural" to me. What's the relative humidity at that location?
    – isherwood
    Feb 3, 2022 at 16:54
  • It is possible something else is using water constantly. For example a toilet with a constant drip. As a result, your water lines stays cold and thus condensation forms. Normally, the pipe should warm up during the day, while not in use, and condensation would be reduced greatly. Check for unintended constant usage.
    – Jeffrey
    Feb 3, 2022 at 18:42

3 Answers 3

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In small quantities like these that would most likely be simple condensation (i.e. water from the surrounding air). Cooler air (near your pipe) can't contain as much water, so that becomes liquid and forms droplets. Nothing to worry about.

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  • I tie a rag round the drippier ones. Gives it just enough time to re-evaporate rather than make what's probably only three drips a day onto the floor underneath - but it becomes one less thing to worry about; water can cut the Grand Canyon if you give it enough time ;)
    – Tetsujin
    Feb 3, 2022 at 10:21
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    I would also isolate the pipe to prevent damage from drops under it, if practical.
    – Orbit
    Feb 3, 2022 at 10:47
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More condensation than sweat! Just like the inside of windows, and sometimes the cold tap in a shower room, that seem to get wet when showering, the moisture in the room is looking for somewhere colder to deposit itself on. That cold water pipe, and particularly the metal joint, are ideal.

Some insulation, like pipe lagging, will mean that moisture will end up somewhere else, but non-absorbent stuff will save that point being the victim.

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  • Comments have disappeared - but - I consider sweating as the act of soldering - joints on pipes. True, Collins dictionary mentions sweating of water on the outside of cold drinks glasses, hence my initial comment.
    – Tim
    Feb 4, 2022 at 9:28
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If this is happening under your shower you could get good results by reducing air exchange between your bathroom and the floor void beneath it. Seal all holes and cracks very well. There should be little to no air flow in the space around these pipes.

It's very common to see condensation on metal cold water pipes inside a bathroom. Look for example at the copper or chrome stub feeding your toilet (if it isn't plastic). The constant high temperature and humidity in the bathroom combined with the cold metal pipe causes this.

You don't want this happening outside the bathroom and especially not inside walls and floors where the condensation will drip and slowly rot your woodwork.

Insulating the pipes may help but what you really want is to eliminate as much as possible any air flow into or out of the floor/wall space and especially any exchange with the bathroom.

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