I live and work in humid Myrtle Beach, SC and have had a number of "mold in the bathroom" calls over the years. Recently, a homeowner called about mold growing on the walls near the ceiling.
The bathroom in question had an exhaust fan, but no supply register. The exhaust fan was installed with a ½-in. gap on three sides where it penetrated the drywall ceiling. The ceiling was 9 ft. high and the door was 6-ft, 8-in. The mold was above doorframe all the way around the room, but mostly on uninsulated walls that were air conditioned on the other side.
The most important cure here is to seal the exhaust fan and any other openings into the attic to the drywall on all sides. I did this with a can of urethane spray foam. I also recommended to the customer that we install a small supply register for that room.
This type of mold is caused by humid air from the attic infiltrating the cool bathroom and condensing on the cool surfaces above the doorframe. Any humid air that gets below the doorframe mixes with the air from the next room, which dilutes its humidity so that no mold grows below the doorframe.
Walls with conditioned space on both sides are the coolest, so they get the most condensation and the most mold. Note that the walls do not actually have to get wet. If the relative humidity gets above about 80% for 48 hours at the surface, it's possible for mold to grow there.
While you're fixing the mold problem, make sure prevailing winds aren't causing the home's bathroom exhaust fans to back-draft. Bathroom exhaust fans should be ducted all the way to the outside, and should have a back-draft damper in the discharge hood, just like in a dryer vent.
Always remember, air conditioning begins with AIR.
Kevin O’Neill, CM, is the co-owner of O’Neill-Bagwell Cooling & Heating, Myrtle Beach, SC. He has 35 years experience in the HVAC service business, is a 28-year member of RSES, and was a finalist in the 2005 NATE Certified Technician Competition. Kevin can be reached at 843/385-2220; e-mail firstname.lastname@example.org.