Q: For more than a year, we have noticed random-sized, pale-yellow fuzzy spots appearing on the bathroom ceiling mostly over and near the shower stall and toilet. Do you have any idea what they could be and how to eliminate them and prevent them from reappearing?

This bathroom was remodeled in 2006 with a tiled shower stall that is open at the top. The ceiling fan is appropriately specified for rooms up to 75 square feet. One double-hung window with opaque glass provides some daylight.

The shower is normally used twice a day. We use the timed fan for showers, and after showering, the fan is turned to the maximum for an additional 30 minutes with the shower door left open.

Both sides of the plastic strip between the bottom of the shower door and curb are sponged dry after each shower. Shower glass walls and all horizontal surfaces are squeegeed daily after the second shower. We have blown-in ceiling insulation.

What do you think?

A: First, thanks for all the information. A little detective work is in order, and this makes the sleuthing easier. Secondly, we congratulate you on your maintenance routine. Regular wall wiping and squeegee use keeps the grout and glass enclosure fresh.

By your description, you have a minor mold problem. The toilet is a non-issue. A weak bleach solution will clean the growth from the ceiling. Now the challenge is to find the cause.

We think it’s ventilation.

While it’s true your fan is rated sufficiently for the size of the room, a fan alone may not be enough. Three things are necessary for mold to grow: food, moisture and warmth. The food source is the drywall. Moisture comes from the shower. Warmth comes from hot water from the shower and the heater in the winter. It’s significant that the fungus is growing above the shower. Warm air is trapped there and is providing the optimum medium for fungus growth.

The solution might be as simple as leaving the bathroom door open and opening the window a bit to promote cross ventilation. Hopefully, this works. But even if it does, you should investigate further.

Poke your head in the attic and make sure the fan is vented to the outside, not into the attic. If the warm, moist air is discharged into the attic rather than the wild blue yonder, you are creating a moist environment conducive to mold growth. Also, check the duct to make sure it isn’t kinked. A kinked duct line reduces the fan’s ability to remove air from the attic.

While you’re at it, check out the ventilation in the attic itself. Hot air rises. As the attic heats up, sufficient air movement is necessary to discharge the warm air. Proper attic ventilation is especially important in the winter months when the heater’s on.

A couple of good-size gable vents are the minimum, but hopefully you’ve got soffit vents and sufficient ventilation at the ridge to move the warm air. If the house is equipped with soffit vents, make sure they are clear of insulation.

To sum it up: Keep using the exhaust fan, leave the door open and crack the window for cross ventilation, and check that the fan is vented outside and that there is adequate ventilation in the attic.

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