Condensation on plumbing pipes, tanks, toilets: here we explain the causes, significance, and cures for condensation or "sweating" on plumbing system components like pipes, water tanks, and toilets. A certain amount of moisture condensation on building plumbing systems is normal in areas of warm humid weather and cold or chilly incoming water supply.
But as we explain here, excessive condensation on plumbing systems can cause costly problems in buildings. The articles at this website will answer most questions about plumbing drain, waste, vent, water supply & septic systems.
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Our sweating toilet tank photo (left) illustrates where condensation may form on the tank of a toilet that is in frequent use during hot humid weather, or on a toilet whose flush valve leaks, allowing the toilet to "run" continuously.
Condensation, the collection of airborne moisture on cool surfaces can happen anywhere in the plumbing system where components are cooler than surrounding, moist air. Common places where we see condensation or "sweating" include toilet tanks, cold water pipes, and water storage or water pressure tanks.
Carson Dunlop Associates in their Home Reference Book point out that in some homes, the cold water piping is insulated to avoid sweating of pipes.
On a warm humid day, cold water running through a pipe will cool the adjacent air, causing condensation on the pipes, tanks, toilets etc.
This ‘sweating’ can be annoying, and if allowed to continue, can damage ceilings, floors, furniture or storage below. Condensation that drips off of these locations can even lead to building rot, insect attack, or to the need for a costly mold cleanup job.
So Do Water Pipes Actually "Sweat" in buildings?
No. Pipes and tanks don't literally "sweat". "Water pipes do not "sweat" as people say - water is not exuding out of pores in the pipe. Water is condensing from moist air onto the surface of the cold water pipe.
Insulate your cold water pipes to avoid condensation and drips onto the floor. What people popularly refer to as "sweating pipes" really is airborne moisture that is condensing out of humid air onto a cool pipe, tank, or other surface.
Where Does Condensation Occur on Plumbing Systems?
Our photo (left) illustrates that "sweating" cold water pipes can be a serious hazard. These cold water pipes are dripping condensate (red arrow at left) right into the electrical panel (down-pointing red arrow at left), risking corrosion of the circuit breakers and other electrical components.
Corrosion can lead to failure of a breaker to trip in event of an over-current. So water pipe condensation could actually contribute to a building fire! Details are
What Causes Condensation on Plumbing System Components like Pipes, Tanks, Toilets?
Why does water condense on your cold water pipes overhead in the basement before it condenses on the steel Lally columns supporting your main girder? It's because the cold water pipe surfaces are colder than the Lally column surfaces exposed to the same moisture-laden air.
When cold water (at 40 deg.F.) is running through the water pipe, the pipe surface is cooled to a lower temperature (40 deg.F.) than that of the Lally column (perhaps 55 deg.F. or higher in a typical residential building basement).
When the temperature of air reaches the dew point (a function of the combination of a particular air temperature and the amount of moisture in the air), moisture condenses out of the air onto nearby cooler surfaces. See DEW POINT TABLE - CONDENSATION POINT GUIDE for more details.
[Note: Another definition of "sweating pipes" is used by plumbers to refer to the process of soldering copper plumbing joints.]
Condensation May Indicate Plumbing Leaks or Private Well Piping Problems
A telltale sign that a toilet is running is the observation of water rippling in the toilet bowl long after the toilet was flushed.
The condensation visible on the toilet tank in our photo (Left) was present even when the toilet had not been flushed for 24 hours, leading to an investigation and cure of a leaky toilet flush valve.
But some toilet leaks are slow and subtle and may not be so visually obvious. But if a toilet is leaky or running, in warm humid weather you may notice that the toilet tank is wet with condensation even when the toilet has not been flushed for several hours or more.
Toilet tank condensation, if chronic and significant, can lead to bathroom floor tile damage, subfloor rot, and even attack by wood destroying insects or mold.
See TOILET RUNS CONTINUOUSLY for details.
Hidden water supply pipe leaks: similar to the case above, a plumbing fixture with a running faucet is pretty obvious. But a hidden supply piping leak may be dripping or leaking into a building wall cavity or even outside or underground.
If the cold water pipes in your building are wet with condensation even though you think no water has been running for hours, there may be a hidden plumbing leak. A great time to check for this condition is on first arising in the morning, before plumbing fixtures have been used.
Our photograph of stains on an interior wall (left) is an example of indoor stains caused by moisture on building plumbing pipes.
Private well piping or well problems: intermittent cycling of a well pump when no water is being run is often a sign of either a running plumbing fixture in the building, or a leak in the well piping itself. If you see condensation on water piping entering the water pressure tank and hear intermittent well pump cycling for no apparent reason, further investigation is warranted.
Indoor stains in ceilings or walls, flooring damage, and even hidden mold are all problems that can be caused by hidden plumbing leaks or by condensation on cold water piping where it passes through building cavities.
Insulate Cold Water Piping
For at least two reasons, that of energy efficiency and to prevent moisture drips and possible mold growth inside basement ceiling cavities, you might want to insulate your hot water and heating pipes in a basement as well, though in some conditions we are so desperate to warm and dry a problem area that we deliberately leave the hot water and heating pipe insulation off of those pipes so that we can steal some of their heat to warm and dry an area.
Carson Dunlop suggest that if a basement is to be finished, the cold water piping above the ceiling should be insulated.
Foam insulating tubes that snap around plumbing piping are widely available and work very nicely for this purpose. We advise against wrapping pipes in fiberglass insulation.
Insulate the Water Pressure Tank?
Where a water pressure tank is located in a finished basement or similar area, having puddles of condensate on the floor around the water tank can be a problem. If you have made sure that there are no plumbing leaks or well pump short cycling problems that are keeping the water tank cold and subject to condensation too much of the time, insulating the water tank with an insulation blanket may be helpful.
Watch out: we have inspected properties where the water pressure tank was located in a finished basement, enclosed in a closet, and surrounded with fiberglass insulation. Wetting fiberglass insulation is inviting a hidden mold contamination problem. Insulating such "closets" with solid foam insulation may be a more mold-resistant approach. See INSULATION MOLD CONTAMINATION TEST for details.
Insulate Toilet Tanks?
Some toilet models currently sold include an insulated toilet tank and may be appropriate if you live in an area where weather is hot and humid for much of the year, as toilet tank condensation can be a problem even if the toilet is not leaky or running. But before going to the cost and trouble of changing out a toilet tank or trying to insulate the exterior of the tank, make triple sure that your toilet is not leaky and running.
Fix Plumbing Leaks
Find and fix running toilets, hidden plumbing leaks, well piping problems or municipal water piping problems that we listed earlier.
Use a Dehumidifier, Add Heat, Increase Air Circulation
Our preferred method for reducing condensation on plumbing pipes, tanks, toilets indoors is to reduce the level of indoor humidity to an appropriate level using either a local portable dehumidifier, or the building's central air conditioning system. See HUMIDITY LEVEL TARGET for details.
In some circumstances increasing the air circulation around a condensation-prone water tank can reduce the total quantity of condensation, at least if the condensation is from normal use and not from a plumbing leak. In some commercial installations and in a few private homes where condensation people add heat where spot condensation is a particular problem.
In the FAQs section just below we include additional indoor moisture collection & condensation problems & suggested cures.
Continue reading at MOISTURE PROBLEMS: CAUSE & CURE or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: what can I do to stop moisture from condensing on the bathroom ceiling & trim?
We have floor ac/heat vents. In master bathroom, we have ventilation fan over the toilet, there is warm air seeping from the attic through the vent and cool a/c coming up from the floor vent. (I think) This is causing the moisture to form on the ceiling and crown molding of the bathroom. What can I do to stop it? We live in very humid Alabama. - L.F. 9/4/2013
Thank you for the interesting question - it helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. You will find additional depth and detail in articles at our website.
That said I offer these comments:
Causes of down-flowing air in buildings
First, it's unusual for warm moist air to flow downwards from an attic or roof space into a bathroom or other room below, and indeed if warm moist attic air enters a cooler indoor space the moisture carried in that air might be expected to condense or collect on cooler building surfaces. Such downflowing air in buildings, contrary to the usual upwards convection currents in buildings, can indeed happen for several reasons, all of which have one of two basic underlying causes. Either
In both of these cases the room is basically "sucking" warm moist attic air downwards from above, with that air passing through any openings it can find, such as around a ceiling vent fan or light fixture.
The cool air coming up from the floor is not itself "causing" indoor condensation, but the cooler indoor temperatures it creates as well as possible downflow of heavier cool air in that room could be factors in the condensation or moisture collection you describe.
How to fix down-flowing moist attic air leaks & related indoor condensation
We finally got it fixed, we added a larger vent fan to bathroom, but that didn't seem to help. We took temp. readings from the floor vents, the temp. of the air was 34 degrees (first vent, closest to AC unit). We closed it almost all the way and that reduced the moisture and (so far) we've had no more problems. Thanks! R.F. 12/4/13
Thanks R.F. for the followup; it sounds as if by reducing the extent of cooling in the bath you reduced condensation that was occurring there. When you cut back on cooling in the humid bathroom you gave the vent fan a chance to exhaust moisture outside, and by warming the bathroom surfaces you're reducing the rate at which condensation forms there.
Question: A/C condensate drips into sink in a nursing home
(May 7, 2012) Lemastre said:
In a nursing home I noted a 3/4" copper pipe protruding an inch or so down from the ceiling and dripping water into the sink directly below. The nursing home uses a tempered-water heating/cooling system, so there are pipes and maybe even a heat-exchanger in the ceiling over each living area. What does a building code usually say about disposing of condensate in such a system?
If the water you see dripping into a sink is A/C condensate there is a risk of legionnaire's disease and the installation is unsafe.
Question: toilet sweats a lot, how can we fix it
(June 20, 2012) Jo said:
The tile flooring in our toilet sweats alot I think this is due to the clay base if it has been raining for some time. How can we prevent this.
Jo, since it is usually not at all cost feasible to chop up a concrete floor to insulate below it, what remains to stop condensation on a cool floor surface is to reduce the indoor humidity level. That means running a dehumidifier as well as finding and fixing any indoor leaks or moisture sources. In a small bathroom where there is not good air circulation, the condensation problem may be worse than elsewhere on the same floor. A small fan that increases air circulation across the floor might also help. But try a dehumidifier.
But first, check for a toilet fill valve or flush valve that's leaking, causing a running toilet - the most common cause of excessive condensate formation on toilet tanks and bowls.
Question: condensate on sewer pipe
(Jan 2, 2013) Diane V. said:
We have cast iron sewer pipe in the basement and it condensates and drips rust-colored drops. The previous owner had wrapped a plastic garbage bag around part of it, but we'd like to know how to keep it from happening. We can't find a leak source. I'd like to move our chest freezer to under where the pipe runs down to make more space in the laundry area, but can't until we get this drip figured out...I don't want it dripping all over the freezer!
It sounds to me as if there is a leak; cast iron can have a hidden crack or on occasion rust perforation. A thorough inspection is needed. I agree that it may be unsanitary
Question: solar panels and attic condensation
(Jan 9, 2013) michael said:
can solar panels cause condensation in attics
Michael that'd be unusual. More likely there is an indoor moisture source or the attic is not properly ventilated, or both.
Question: aquarium heater used in toilet to stop condensation?
(1 days ago) Nicholas Alipaz said:
I was able to quickly solve my toilet sweating problem by placing a small standard aquarium heater into the tank. I have an outlet just below my toilet to plug in the heater, so it was quite easy. The rule for these heaters however is to always keep them immersed in water to avoid breaking them, however I found that the very short amount of time whilst the toilet tank is filling with water after a flush has no visible ill effect on the heater. I have had the same heater in my tank for 2 years and it is still going strong.
(1 days ago) DanJoeFriedman (mod) said:
Interesting idea Nicholas. I would take the solution to toilet tank just one step further and make sure that the toilet is not running. Sometimes that sort of leak is ot immediately obvious. We don't want to fix joist the symptom if there is an underlying problem to be addressed. For example a running toilet wastes water and floods septic leachfields.
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