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AC causing condensation on bathroom wall

Hello All,

Thanks in advance for your advice.

My house is about 35 yrs old.. Concrete Block construction in South Florida...

We noticed mold growing on the wall above the tiles in the shower.. to the touch, the wall is always cold, I noticed that the AC runs in the wall behind the bathroom, this causes condensation to form on the wall, along with the moisture from the shower, the wall stays wet. The bathroom does have an exhaust fan.

I cleaned the mold with bleach, primed with Zinsser Bulls Eye 123, and painted. 2 weeks later, I'm noticing drip lines down the tile in the color of the paint...

What can I do to keep the condensation from ruining the paint, wall and avoid mold from growing.

Re: AC causing condensation on bathroom wall

First off I would try.....Acquiring and mounting an exhaust fan suitable for mounting in the ceiling directly above the shower/tub. Exhaust it to the great out-of-doors (not to the attic space). Use whenever showering and allow to run 10-15 minutes after you're done.

Re: AC causing condensation on bathroom wall


There is already an exhaust fan in the bathroom, vented to a soffit vent to the outside. The BR is very small, about 6x6 or so. My concern is the sweating wall not the added moisture from taking showers. It seems that if the AC is running all day (which it mostly is) condensation will form on that wall due to the cold air passing behind it.

Re: AC causing condensation on bathroom wall

Am guessing that the duct is not insulated. Any chance you can yank the ductwork and replace with insulated duct?

Do you notice any condensation on the tiles as well....or only on the the wall above the tile?

If only on the wall above, then it would seem that the extra material behind that tile and the tile themselves are providing just enough more r-value that condensation does not occur. If so, this would indicate that very little more insulation would be necessary to prevent condensation above the tile as well.

I'm still inclined to think that better exhaust ventilation would be helpful in this matter. Is there an adequate source for make-up air while this fan is running? Door undercut 5/8" or more? If the vent fan isn't effective because of no make-up air or similar....water vapor will collect on or penetrate all the surfaces in the room. That water vapor will later dissipate, raising the humidity level in the room which means more WV to condense on that wall area in front of the AC duct.

Was this exhaust fan installed with flexible plastic duct or rigid dutwork? Flexible duct obstructs the flow rate much more than rigid does. How far does the duct run before it exits the house? Too long of a duct and the rate of discharge drops considerably.

Is there a return duct for the AC in the bathroom also or a supply only? If no return then circulation is likely inhibited to some degree. Maybe substantially so. If the circulation is adequate and the AC is running nearly all day, it seems that the RH of the room should drop to the point where it is in sync with the rest of the house. If that's the case, then you should see condensation in front of the other AC ducts runs also....but you report the bathroom only. See where I'm going with the suspicions of a higher RH in that room?

Or....you might try leaving the door open to the bathroom at all times and running a fan to circulate more air thru the room.

Or.......maybe this wetness that you see above those tiles is actually soaking in from the backside of the drywall (or whatever is back there). The WV may be condensing on the AC duct inside the wall and is supplied by high humidity from the great out-of-doors. If so, this would indicate that either that stud bay that houses this AC duct needs to be better sealed or the duct will need to be moved to an interior wall. Is it perhaps the only AC run in the house that is in an exterior wall?

On that note - Does this duct run up the wall and then discharge from the ceiling? Anybody crawl up into the attic space to see if the stud bay is well sealed around the duct where it emerges? How about where the duct first enters this stud bay? A can of foam may be the answer to sealing this bay to prevent outside WV from entering and condensing on the duct inside the wall.

Another question - You say the house is 35 YO. House is new to you ...or has this just problem just started? If it just began, that would indicate that something has changed. Disintegrated insulation around the duct? Or...material that used to seal up this bay around the duct where it enters or exits has disintegrated. ???

About impossible to diagnose from here, but I hope this helps you diagnose and remedy the problem.

Re: AC causing condensation on bathroom wall

Thanks for all thee great suggestions and ideas.

House is new to me, 2 yrs now.

The exhaust fan was just installed last week. Hampton Bay fan, 3" to 4" adapter, 4" Semi Rigid duct runs about 3' parallel to the ceiling to an insulated soffit in the eve. The duct had to be bent into a flat position to fit into the eve, however I was still able to get my hand in there, so I assume airflow is good. If I run the fan, I can feel airflow outside.

I'll check on the undercut of the door to increase air circulation. There is no return or supply for AC to the bathroom limiting air flow.

Back to the wall... Wall is only wet to the touch above the tile line. If the AC is off for a day or so, there is no sweating on the wall. Also, drywall seems solid to the touch & knock. Doesn't appear to be soft or swollen. I will post some pics when I get home, as I fear I may not be explaining properly.

AC Unit is on the side of the house, duct comes into the house through the Garage. Opposite this interior wall which houses the duct is the bathroom, with the sweating wall. Design of this duct:(layman terms) comes in to the garage thru exterior wall at about knee level.. goes vertical to about 6-8 feet, then horizontal about 10 feet where it meets with another vertical duct which has the return at the bottom. In the attic, I see the top of this section which splits off in all directions using round duct. In the garage, the Duct was framed with studs and drywall. The space below the ducts was used for the washer/dryer.

Attic is mostly crawlspace as you can only really get into the peak... everything is is crawling across studs trying not to fall through the ceiling.. insulation is blown fiberglass.. so there is pink crap all over. :)

I think I will have an AC company come check it out.. Hopefully this won't cost an arm & leg to diagnose & fix.

Thank you Goldhiller.

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