Mold on sheetrock question...
 

I've got a question for the brain trust here at PPOT.

Got a homeowner in a house that's just a few months old. A small amount of mold has started showing up on the hall bath ceiling.

This is a secondary bath where they give their 14 month old twins a bath in shallow water twice a week.

There are no water stains, wet seams, or evidence of water coming from above. The Sheetrock is bone dry in the attic, as is the insulation.

The hall bath is divided into two sections, with a door between them. One side has the double vanity with an HVAC vent in the ceiling. The other side has the toilet and tub with two lights and a fart fan.

Any idea what might be causing it?

Mold spores?
;)
Perhaps a missing or damaged vapour barrier.

Mold by fan? Duct leaking, or condensing in exhaust pipe.

That's a tough one... no plumbing or leak up above and the area is rarely used.

If it's coming from behind the paint the drywall must have been wet at some point prior to installation and the water in the paint was enough to activate the mold.

If it's in the half with the a/c, I'd check what the relative humidity in the bathroom normally is and whether the a/c register is blowing in a way so that it creates a cold spot on the ceiling where condensation is able to form.

If it's in the half of the room without the register, if the trouble is near the fart fan or lights air from the attic/outside could be infiltrating in and condensing on the first cool surface it comes in contact with. The dew point has been between 75-80 for two months now, and living space is cooled to below that level.

A quick spritz with bleach will kill the mold.

One coat of shellac, one coat of paint.

Wire the fans to come on with the light switch.

They need to move the air. They need to run the fart fan to pull the wet air out of the room after the kid gets a bath. Leave the door open too.

Also run a dehumidifier for a while and see what amount of water is collected. What kind of ventilation is available?

do not screw with mold. it is toxic and needs to be addressed. Licensed professionals should cut out the affected area, dry it, treat it with bleach or other products and then fix the source. You do not get mold without water present. It will also grow horizontally quickly and be hard to eradicate. This is a real problem if there are kids present

What type of paint is on the ceiling? Should be interior semi-gloss. If painted with flat the paint will absorb moisture and mold will form quickly.

The house is only a few months old so I doubt this has turned into a major issue needing professionals. I would stop using the bath for a couple weeks, treat the area with bleach a couple times and see what happens. Complete with proper paint.

Previous owner might have painted over the existing mold and now it coming through. Once its in the drywall or plaster, it tough to get rid of it unless the piece is removed and drown it with bleach.

It's interior condensation related to the use of the bathroom/bath. No need to be worried. The warm moist air from all the uses of the bathroom is rising to the ceiling where it gets trapped, condenses, and the water and warmth support growth of something on the painted surface. If you check the humidity in those areas you'll find it is too high. The solution is more air transfer and lowering the humidity. You should install a bigger exhaust fan in the area and they should try to minimize using water in a way that encourages it to evaporate. That means shorter showers and not using as much hot water.

Your mold abatement is simple. Wipe the ceiling clean with a rag and cleaner, the way you'd clean anything else. For added comfort you can wipe it with a 5% solution of bleach. That's about a tablespoon of bleach in a half gallon or so of water. If you can smell the bleach it's enough.

What others said, about semi-gloss, and air movement. Get a moisture meter, just because it feels dry, doesn't mean it is. Case in point, the mold needs moisture to grow. 5-10% bleach in water, no more than that.

This is a brand new house and it's only in this one side of the bathroom. They said it just popped up one day.

The ceiling is painted with a flat latex paint. There's a can light over the tub and a fan light combo over the toilet.

The mold is not severe but you can see it in several places.

As this is the hall bath, it's only used to bath the two small children. No hot showers.

HVAC guys couldn't find anything to point towards the problem.

I'd like to find a problem before we bleach it, kilz it and paint it. Then see if it happens again with the fan running during and for twenty minutes after each bath.

It's condensation. For some reason the air flow is bringing the warm moist air from the bath, kitchen and rest of the house into that section of the hall bath. It's just a quirk of how your HVAC system came together. There's nothing wrong with it, you just need more ventilation in the area.

People consume hundreds of gallons of water every day. Everything from bathing and drinking to cooking and breathing. I've seen the data on how much water vapor an American puts into the air just in his daily life. I don't remember what the specific numbers were, but it's remarkable. For some reason the air currents flow into that area and it gets more humid there than anywhere else. Being summer in SC doesn't help matters.

The house probably needs a whole-house air exchanger if it doesn't already have one. If it does, it needs adjusting. But just get a better fan in that area and the problem will go away.

where is the fan vented to?

The flat paint is absorbing moisture. Bathrooms should be painted with a semi-gloss.

Summer is rough here. My place is under 10 miles from the suburb the OP is in and adding a whole house dehumidifier is one of the best decisions I've made. Controlling air infiltration helped a lot too, and encapsulating the crawlspace probably didn't hurt either. Going from 65 to 50% relative humidity indoors made a world of difference.

It's vented to a soffit outside. It's a type that doesn't have a flap, but more of a diffuser and slots for ventilation.

There is a flap on the fart fan, but the way it's set up/balanced, it's open maybe 5-10%. I've thought about glueing a penny on it to close it 100% just in case moisture was coming in from the exterior. I just don't see it making it through 10-20' of 3" duct.

One thing the HVAC tech told was the HO had the thermostat set at about 66*. With a temp outside of mid 90*'s, it's going to run and run trying to get down to the set temp. It's also going to cause the air handler in the attic to sweat bullets and eventually cause a mold issue on the air handler. There was a cup of water in the overflow pan and the bottom of the air handler was wet.

None of that was reaching the bathroom clg though.

Might increase the fan size to move more air. As well, I'd go with a semi gloss on the ceiling in a bath. More moisture resistant. Forget the whole " mold is evil " thing , just kills it and paint over. As far as the air handler thing just make sure the drains run freely and all is well no matter the t-stat setting.
Mt Pleasant? My ex lives there.... Puts the Mount in Pleasant she does :) better there than here