Answers / Problems & Symptoms

Mold in an Apartment

Urgency: Moderate Last updated September 27, 2026 · Monitor My Air

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Most likely causes, in order

1. Bathroom fan missing, broken, or venting into a ceiling cavity instead of outside, so shower moisture stays in the unit.
2. A leak from the unit above, a shared wall, or a common plumbing stack, showing up as a stain that grows after their showers or laundry.
3. A wall or window air conditioner or PTAC with a clogged condensate drain or a dirty coil, dripping into the wall or floor below it.
4. Condensation on a cold exterior wall or single-pane window in winter, usually behind furniture pushed against the wall or in a closet on that wall.
5. Indoor humidity kept high by drying clothes indoors, cooking without a hood, or a unit that never gets outdoor air.

Mold in an apartment is the same moisture problem as mold in a house, with two twists: you did not build the place, and someone else usually has to fix the building side of it. Here is how to work out what is going on and who owns the fix.

Why apartments get mold

An apartment has less exterior wall than a house, so the wall it does have gets cold and sweats in winter. Bathrooms are small and often vented into a ceiling space instead of outdoors. Plumbing is shared, so a leak in the unit above lands in your ceiling. And the heating and cooling equipment is frequently a wall unit or a PTAC with a drain that nobody has cleaned. Any one of those puts water where it can sit, and mold follows within days.

Whose problem it is

Find the moisture source first, then the responsibility usually sorts itself out. A leak from above, a fan that does not vent outside, a failing window seal, or a wall unit that drips are building problems and belong to the landlord. Humidity from drying laundry indoors, showers with the fan off, or furniture jammed against a cold wall are use problems and belong to you. Most cases are a mix, which is why documenting what you find matters more than arguing about it.

Documenting it

• Dated photos of the mold and of anything wet, stained, or dripping, with something for scale.
• A week of readings from a hygrometer ($10 to $15) placed in the room, morning and evening. Above 60 percent for most of the week points at humidity; readings under 50 percent with mold still growing point at a leak or condensation.
• The date you told the landlord or manager, in writing, and what they did.
• Any symptoms that improve when you are away from the unit and return when you are home, with dates.

Approximate costs

• Hygrometer to read humidity: $10 to $15
• Moisture meter to check a wall or ceiling: $25 to $40
• Dehumidifier if the readings stay above 60 percent: $200 to $350
• An independent inspection, if the landlord disputes the cause: see the cost guide on this site for typical ranges

When to call a professional

• The mold keeps returning after cleaning, or covers more than about 10 square feet.
• There is a stain that grows, a soft wall, or a musty smell with nothing visible.
• Someone in the unit has breathing trouble that improves away from home.
• The landlord and you disagree about the cause. An independent investigation that documents the moisture source settles it, and the report is written to be read by both sides.

What not to do

• Do not paint over it or spray bleach on drywall. Bleach does not reach mold in porous material and adds water.
• Do not wait for the next lease renewal to raise it. Written notice with photos, dated, is what protects you.
• Do not withhold rent or break the lease on your own reading of the law. Tenant rules differ by state and city; get local advice before acting.

Related answers

• Bathroom Smells Musty After Shower
• Mold in Closet
• Water Stain Appeared on Ceiling
• Musty Smell in Bedroom
• Mold Smell but No Visible Mold

Education, not medical advice or a professional inspection of your property.