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Started By
Message
Bathroom ceiling is moist with water bubbles forming in drywall; persistent mold issues
Posted on 7/27/26 at 2:21 pm
Posted on 7/27/26 at 2:21 pm
Our guest bathroom is some kind of enigma.
Some notes on the design:
It is a small bathroom that is now a jack and jill after previous owners built an addition. It has a linen closet on the interior of the bathroom. There's no door on the closet. The toilet is near the wall that was once the exterior of the house. Right above the toilet is the bathroom exhaust fan (50cfm). The exhaust fan has not been connected to a duct to ventilate outside. Outside the opposite side door of the exhaust fan is the HVAC return. There's also an air vent for air conditioned air to enter the bathroom.
This bathroom has had persistent mold issues, notably in the linen closet. The ceiling of the closet gets black mold and milder all over within a few months of cleaning. We hired a mold specialist who did some super expensive cleaning that had little effect. We paid an insulation company to suck out all the old insulation, vacuum and air seal the attic, and blow in fiberglass R30.
We've tried having our kids shower with one of the doors open. We've tried keeping the doors closed with the exhaust fan running and the a/c pumping. We've tried painting with Killz. We paid the mold specialist. We've insulated it.
My last hope here is that having the bathroom exhaust fan moved to the center of the bathroom and boosting the cfm to 110 with a proper ventilation through the roof will take care of it.
But I don't have much hope.
The HVAC guy says we should get an attic fan installed because the attic is too hot. But when I use an IR, the ceiling is registering 76 degrees at the hottest part of the day. The bathroom exhaust fan is registering 100 though. But it's such a small square spot. Why would there be moisture issues on the other side of the bathroom?
So why is so much condensation building up in the bathroom?
Any thoughts or ideas?
Some notes on the design:
It is a small bathroom that is now a jack and jill after previous owners built an addition. It has a linen closet on the interior of the bathroom. There's no door on the closet. The toilet is near the wall that was once the exterior of the house. Right above the toilet is the bathroom exhaust fan (50cfm). The exhaust fan has not been connected to a duct to ventilate outside. Outside the opposite side door of the exhaust fan is the HVAC return. There's also an air vent for air conditioned air to enter the bathroom.
This bathroom has had persistent mold issues, notably in the linen closet. The ceiling of the closet gets black mold and milder all over within a few months of cleaning. We hired a mold specialist who did some super expensive cleaning that had little effect. We paid an insulation company to suck out all the old insulation, vacuum and air seal the attic, and blow in fiberglass R30.
We've tried having our kids shower with one of the doors open. We've tried keeping the doors closed with the exhaust fan running and the a/c pumping. We've tried painting with Killz. We paid the mold specialist. We've insulated it.
My last hope here is that having the bathroom exhaust fan moved to the center of the bathroom and boosting the cfm to 110 with a proper ventilation through the roof will take care of it.
But I don't have much hope.
The HVAC guy says we should get an attic fan installed because the attic is too hot. But when I use an IR, the ceiling is registering 76 degrees at the hottest part of the day. The bathroom exhaust fan is registering 100 though. But it's such a small square spot. Why would there be moisture issues on the other side of the bathroom?
So why is so much condensation building up in the bathroom?
Any thoughts or ideas?
5
Posted on 7/27/26 at 2:26 pm to StringedInstruments
I think your idea of venting thru the roof is a good idea.
Not sure of the shower head you're using, but I'd have one with low flow to keep the steam down.
Not sure of the shower head you're using, but I'd have one with low flow to keep the steam down.
Posted on 7/27/26 at 2:30 pm to StringedInstruments
Sounds like a leak from the toilet vent pipe, that typically goes up through the roof. Perhaps you should re-flash and seal it, and then remove some of the water damaged drywall to see if that fixes the cause.
And here is a crazy theory. Since your HVAC is in the attic close to the bathroom (with the return right there), go into the attic and examine the A/C condensation drain pipe. There is a chance that the condensation from the A/C runs in a 3/4 inch pvc pipe to drain into the sewer vent pipe for the toilet where it passes through the attic. If so, one of the joints on the pvc drain pipe may be leaking, thereby leaking A/C condensate into the sheetrock above the bathroom.
The only reason why I include the second theory is that this actually happened at our old house. The A/C drain line that drained from an A/C in the attic drained into a sewer vent pipe above our hall bathroom. It got clogged-as they all do sometimes- and the A/C guy blasted the clog with compressed air. That separated a joint above the bathroom that led to mold and some reconstruction.
And here is a crazy theory. Since your HVAC is in the attic close to the bathroom (with the return right there), go into the attic and examine the A/C condensation drain pipe. There is a chance that the condensation from the A/C runs in a 3/4 inch pvc pipe to drain into the sewer vent pipe for the toilet where it passes through the attic. If so, one of the joints on the pvc drain pipe may be leaking, thereby leaking A/C condensate into the sheetrock above the bathroom.
The only reason why I include the second theory is that this actually happened at our old house. The A/C drain line that drained from an A/C in the attic drained into a sewer vent pipe above our hall bathroom. It got clogged-as they all do sometimes- and the A/C guy blasted the clog with compressed air. That separated a joint above the bathroom that led to mold and some reconstruction.
This post was edited on 7/27/26 at 2:39 pm
Posted on 7/27/26 at 2:38 pm to TBoy
quote:
Sounds like a leak from the toilet vent pipe, that typically goes up through the roof. Perhaps you should re-flash and seal it, and then remove some of the water damaged drywall to see if that fixes the cause.
Yep, Start in the attic and look at the plumbing vent stacks, light coming around them means water is leaking around them into the attic
Posted on 7/27/26 at 3:05 pm to TBoy
quote:
And here is a crazy theory.
I don't think this is that crazy when OP is talking about water bubbles.
And when checking, if the pvc appears dry to the touch wrap it in toilet paper and check the next day. If it gets wet at any time during the day, it'll be obvious.
quote:
the ceiling is registering 76 degrees at the hottest part of the day. The bathroom exhaust fan is registering 100 though.
Consider finding some insulation that you can lay around and on top of the vent.
Also, not all bathroom vents need to vent out. I do not agree with this and have been working on having all of mine venting through the soffit; 1 bathroom left to do, just a pain in the arse to get to it so waiting until I do some other work in there.
Posted on 7/27/26 at 6:31 pm to StringedInstruments
Why not rip out the nasty sheetrock and find out what the problem is?
No point in cleaning that nasty shite. Solve the problem and stop attempting to treat the symptoms.
No point in cleaning that nasty shite. Solve the problem and stop attempting to treat the symptoms.
Posted on 7/28/26 at 8:36 pm to StringedInstruments
I had the exact same issue in a small bathroom. Finally bought a small dehumidifier off Amazon and keep it running 24/7 in the bathroom and it has fixed the mold issue.
Posted on 7/28/26 at 8:45 pm to TBoy
The AC condensate pipe doesn’t even have to have a leak. The cold condensate inside can cause condensation on the outside of the pipe from hot, humid attic air. This then drips onto the ceiling.
Posted on 7/28/26 at 10:09 pm to StringedInstruments
quote:
HVAC guy says we should get an attic fan installed because the attic is too hot
Powered attic fans almost never a good idea.
Posted on 7/28/26 at 11:16 pm to YahooSerious
quote:and far more common than a leak
The cold condensate inside can cause condensation on the outside of the pipe from hot, humid attic air.
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