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Question DetailsAsked on 12/4/2017

Mold on bathroom ceiling. Dry wall was removed to reveal rusted cast iron pipes but no leak. can they be the cause

This is 12th floor of 1985 condo. Should dry wall be replaced if condo won't attend to rusting? They say toilet water evaporation caused mold. We do have vent, ac, etc. Appreciate your thoghts and recommendations.

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3 Answers


I don't buy it - at least not the toilet evaporation argument. if that is true, then the bathroom does not have legal ventilation - though bathroom mold can easily form from failure to remove the airborne moisture from showers, and from long baths.

While quite old cast iron pipes will develop a light rust coating over many years (1985 is certainly old enough for a good thin rust coating to form on the pipes), if it was heavy or scaling (loose or peeling off), or appeared only at certain places on the pipes (usually the bottom except at specific cracks or leak points, then it was from a leak. Rarely unless over about 70 years old from rust-through from the inside, usually from a cracked pipe or leaking joint. If just a uniform coating of thin rust all over, could be from condensation non the pipe or poor ventilation in the wall cavity, not a leak.

Also - if there was enough of a leak to form mold on the bathroom ceiling, if a leaking pipe was the problem there WOULD have been more significant mold/mildew and heavy water staining on the back of the drywall (the side toward the pipe) and almost certainly water staining (orange or yellow halo around the wetted area) on the inside surface of the drywall in addition to the mold. Without that water staining, I would say if there is mold only on the bathroom side of the drywall, it is due to inadequate bathroom ventilation. And paper backing on the back side of the drywall would be fuzzed and likely peeling or wrinkled if it got wet from a leaking pipe.

If the drywall is not water stained, then bathroom moisture is likely the cause - most commonly at the top of exterior walls in winter (because they are colder) and sometimes on ceilings that are cold (attic above). Solution is running bathroom ventilation fan for 20-30 minutes after showers and baths (till mirror is condensation-free at least). Keeping toilet lid closed also helps, as does wiping off the mirror after hot bath/shower if it is heavily condensed up or running water off it. Wiping the water off your body before getting out of bath/shower also helps, because then the towel is not soaking wet when hung up to dry, so less water is put into the air.

Bathroom door should be shut of mostly shut when fan is running so most of the air being removed is from inside the bathroom, not short-circuiting air from an open door. But bathroom door should normally be open otherwise, to allow natural ventilation and to remove the relatively small amount of water evaporating from toilet and drains.

Answered 2 years ago by LCD


I agree with the other answer that this is not from "toilet evaporation". My guess is that if there is no leak then the residual moisture post shower/bath is responsible for mold growth. You do not need a flooding condition or a leak to generate mold. Molds have different moisture requirements, some low moisture requirements and some with higher requirements. If you are depending on evaporation in a bathroom that has limited ventiation then you can expect mold on the ceiling. Wheather that moisture penetrated sufficiently for it to effect the back side of the drywall depends on duration and frequency. Usually in condo situations the drywall will be thick enough to limit the possiblity of penetration. The cast iron soil pipes typically rust over time and while they can leak it is usually after a significant period of time.

Answered 2 years ago by MoldNMore


I was in a bit of a hurry when I did my first response - here are a couple of other diagnostic ways to get a clue to what is causing this mold:

1) if only occurs in linear form right over where the pipe is in the wall, then the pipe would be the expected source of the moisture. But would be rare for that to happen without direct water contact to the drywall or direct dripping/sprayingn on it, which would then be expected to produce water staining as well. While a generally high humidty environment can form mold on drywall from moisture coming from behind (like with damp but not wet basement foundstion behind drywalled studwalls), generally speaking the evaporation capability at the surface of the drywall so far exceeds the amount of moisture which can pass through the moisture-resistant paper and paint and the drywall that if only moist air is behind the drywall (like from a wet but not-in-direct-contact and not dripping on the drywall pipe) not enough moisture would get through to cause mold on the drywall surface unless the bathroom was consistently quite damp anyway or had vey low air exchanges.

2) the general surface rusting of cast iron (or other metal) DWV (sewer) pipes, where not streaked and there is no visible buildup of minerals or sewage slime on the outside from an actual leak, is generally not from inside of the pipe - it is from the cold air (night and winter) passing through the pipe stack up to the roof vent, or sometimes on cold nights cold air dropping down into the pipe by gravity, which drops its temperature and can bring the surface below the dew point, causing condensation on the pipe - and hence formation of a thin, fairly uniform rust over the surface. Can also happen in walls where the pipe penetration into the attic wasnot sealed up, sometimes leaving a substantial opening through the top of the wall which lets cold air pour down in from the attic (or in your case possibly down a pipe chase from attic or roof), cooling the pipe and walls and causing condensation of moisture fromthe building air it contacts.

3) if the mold is concentrated at the top of outside walls only (at the top because the warmer, moister air rises in the room), then it is usually because the outside wall is cold enough that it is dropping below the dew point in the room - which for a moist bathroom, especially after showers, can be well within the normal temperature of the room. Hence condensation on mirrors, which are a bit cooler when wet because they evaporate rapidly, dropping their surface temp relative to the room, so they are usually the first thing moisture condenses on in a bathroom - hence the "fogging" of the mirror. Next, cold ceilings and outside walls see condensation - in serious cases (which can occur at temps as high as 80 degrees or more with really hot or long or steam showering - think about the "fog" and heavy wall condensation in saunas and steam rooms are temps around or exceeding 100 degrees), to the point of water running down the walls and leaving (usually) fainter streaks on the paint, with little brown globs at the end consisting of dust and dissolved paint resin.

4) if the mold is concentrated around light fixtures, light switches, or through-wall/ceiling fans, then it is usually because cold air is coming through there and cooling them or because they are cold because they penetrate the wall and metal is exposed to the cold air on the other side, causing condensation - so they need sealing up around them and possibly a thermal break between inside and outside (one reason plastic fan houseing are better for this problem, plus they do not rust).

Ceiling/wall fans can also see this happen because of the exterior vent flapper sticking open or dropping out, so you are getting significant reverse air flow or cold air.

Uninsulated exhaust fan ducting (especially metal) in the attic can also cause cold air dropping down through the fan.

You can also get mold around an exhaust fan because of condensation in the cold ducting, which can also cause water staining around the fan as the condensate runs down the ducting through the fan into the room - especially common with kitchen over-stove fans after boiling/frying food on cold days, or when using it and putting stove heat up through it after days without use, so condensation/ice from the household air flow up through it and cause it to drop down to the stove through the fan.

Of course, a leak in the roof/wall vent running down through or along the outside of the exhaust duct can also cause water staining and occasionally mold inside at the fan, but usually water running in from outside or from condensation inside the duct is sporadic so can cause staining but more rarely mold. Mold is usually a sign of consistent, repeated and fairly constant wetness for some duration because it stops growing and turns fainter shade (though does not die off) when it dries out, so active mold is almost always a sign of excess moisture in the air, or constant wetness in the wall. Generally (not universally), as moisture levels and consistency goes upwards, the mold type goes from white or gray mildew to borwn mold to black mold to green mold - but varies by locale and mold spores available for growth.

BTW, mold spores are always in the air so you cannot "kill the mold" - you have to control the moisutr levels on the surfaces - have damp surfaces and generally, with relative humidity above about 40-50% and temperatures in about the 40-100F range, it will gorw to at least some extent, just using the mold spores out of the air. Hence, wiping down bathroom surfaces about weekly (after any current mold is thoroughly killed with bleach or such, including the roots embedded in the drywall) is commonly the best way to control it.

Of course, in addition to running bathroom fan for 1/2 hour or more after hot or long (hot obviously worse as it generates warm air which also can hold more water vapor) bath or shower, leaving the bathroom door open when not in use can help provide natural ventilation to dry it out, as can putting wet towels into the laundry immediately rather than hanging them in the bathroom, drying out wet bathmats rather than leaving them on the floor, hand-squeegeeing (yes I looked it up, that is the correct spelling for the action of using a squeegee or hands to rub water off something) your body before getting out of the shower/tub so the bathmat and towels have less water than otherwise, and especially not hanging up dripping wet towels - a common problem with long-haired people wrapping a towel around dripping wet hair for awhile before they dry it with a hair dryer then hanging the dripping wet towel upto dry in the bathroom. Ditto to wet towels from drying a pet - try to dry them on a clothesline outside if possible, or route direct to washer/dryer. In humid warm weather, opening a window (if there is one and it does not present a security risk) when the bathroom is not in use will also help UNLESS your air conditioning temp is low and the outside temp and humidty are very high - in which case the outside air can end up condensing in the colder bathroom.

Of course, because air conditioners reduce humidity in general, assuming you do not have a humidifier on the unit, an air conditioned bathroom (especially if it has an A/C exhaust vent in the room) will have less of a problem than one without A/C.

Additional heat is an iffy thing - if there is bentilation to remove the moisture in the air then a warmer room will evapiorate the moisture on surfaces quicker, but if the humidity level in the room is not reduced by A/C or ventilation, then a higher temp will promote mold growth, so can be a two-edged sword.

Good Luck - and in addition to killing the mold after cleanup, and priming with a mold and stain-preventative like Kilz, I recommend that a mold-resistant additive like CrudKutter or Add-2 powder be added to both the primer (even though Kila already has some) and to the finish coats - though with pure white sometimes it discolors it a bit so that can be an issue in that case.

Answered 2 years ago by LCD

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