Published on August 28th, 2012 | by Henri


Black mold on bathroom ceiling

Q.  Can you give me some help with a problem of recurring black mold on a bathroom ceiling? The house was built in the 1930s and originally had only a tub in the bathroom. There is also a single window in the bathroom. At some time in the past a shower was added over the tub, but no ventilation fan. Hot steam now builds up after a shower. This is especially bad during the winter months when the window is closed.

I have three questions. The first one is what is the best type of ventilator fan to add, i.e., ceiling vs. wall, what size, etc.? The fan obviously must vent to the exterior. Second, what is the best way to clean up the ceiling at this point? Third, is there a special paint that can be used to retard or eliminate the recurrence of the mold once the fan is installed and the mold killed?

A. One simple way to ventilate the bathroom is to open the window when you are finished showering and are ready to leave the room; be sure to close the door behind you. Fifteen minutes should be enough to dissipate the moisture.

But if you decide that this is not enough and you want to install a fan, select a good-quality bathroom fan that is rated as quiet as possible.

Its location depends on whether the bathroom is on the first or second floor. If it is on the first floor of a two- or three-story house and there is a basement, the most energy efficient way (if it is possible) is to install the fan in an interior wall and run a duct to the basement and out a band joist. The reason for this is that when the fan is not in use, the stack effect in the house (warm air exfitrating through upper cracks and cold air infiltrating through lower ones) keeps the outside jack flap closed. Whereas with ducts in ceilings or attics, the wall jack’s flap is pushed open by warm air exhausting through the duct, resulting in constant heat loss.

If the duct cannot be run down to the basement, the fan can be installed in the ceiling and the duct run between two joists to the band joist. If the bathroom is located just under an attic, the duct can be run into the attic and out to a gable wall. In this scenario, it is best to insulate the duct to minimize condensation.

If at all possible, use solid plastic ducting (such as schedule 20 drain pipe) instead of the flexible vinyl type so often used. Also use an aluminum exterior wall jack instead of a vinyl one; the latter are prone to breaking in cold weather.

To clean the bathroom ceiling of mildew, wash it with a solution made of equal parts fresh Clorox bleach and water with a little detergent added. Zinsser makes Perma-White, a paint formulated for bathroom ceiling that is guaranteed to resist mildew for five years.

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