Insulate the attic space above the bathroom to prevent excessively cold surfaces on the ceiling.
Mold in attic above bathroom.
Dryer vents plumbing vents kitchen or bathroom fans vented into the attic.
Moisture food and warmth.
Related to 3 above this practice can pump warm moist air into the attic where it can be trapped and begin the cycle of mold growth.
Paint the bathroom ceiling with a high sheen gloss paint.
Matte and other low sheen paints absorb much more moisture which in turn leads to mold growth.
The most common cause of attic mold by far is blocked or insufficient ventilation of the attic space.
Mold needs 3 things to grow.
Notice how the dark line on the insulation in the photo below line up with the top plate.
If you eliminate just one of these elements mold will starve and die.
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This is why you rarely find mold growth in the attic above a garage or a rarely used portion of the home.
Attics usually have a passive ventilation system in which outside air comes in through the soffit eave vents at the bottom warms up in the attic and escapes through the can or ridge vents at the top because hot air rises.
This is a different perspective from the other video about bathroom fan venting causing mold.