Mold in Bathroom: Find the Source, Size the Fan, Escalate

A bathroom is six or seven surfaces sharing one air volume and one fan, and the surface that grows mold first tells you what is wrong with the room. This page reads the room; the surface pages on our mold hub do the scrubbing.

Which surface is it on? The room map

Where it is	Usual source	First check	Go to
Grout and caulk in the shower	Daily water and soap film	Wipes off tile, stays in the joint?	mold in shower
Ceiling, densest over the tub	Condensation or attic venting	Even, striped or ringed pattern?	mold on a bathroom ceiling
Vanity floor and back panel	Supply, trap or faucet drip	Damp with the taps off?	mold under the sink
Curtain or liner hem	Water held in folds	Bottom few inches worst?	shower curtain mold
Tank, rim or toilet base	Standing water, sweat or seal	Floor soft at the base?	black mold in the toilet
Wall beside the shower	Steam, or water behind tile	Damp on a day nobody showered?	water behind tile
Fan grille and housing	Duct condensation draining back	Holds a tissue?	mold on a bathroom ceiling

What you are looking at in a bathroom

Two of the commonest bathroom stains are not fungi. A damp white cloth, a flashlight held flat to the surface and a dry day sort most of them.

What you see	Most likely	Quick check
Pink or orange slime on tub, shower floor or toilet rim	Serratia marcescens, a bacterium, per NDSU Extension	Smears off; returns in wet corners
Black or green specks on paint, caulk or grout	Surface mold	Raised, fuzzy, smears on a damp cloth
White gritty crust	Mineral or soap scum	Will not smear; dissolves in an acid cleaner
Straight grey lines at joist spacing	Ghosting at framing	Flat and dry; see the ceiling page
Brown ring with a hard edge	Water from above or behind	Damp after a dry spell

NDSU Extension says the pink film feeds on soap residue in tubs, showers and toilets and comes off with a chlorine bleach cleanser. Black is only a colour; either way, fix the water, then clean or remove the material.

Which water is feeding it

Steam and splash are tied to showers; a leak is not. Leave the room unused for a day or two with the door open, then look again.

Source	Pattern	Timing	Fix
Shower steam	Even speckle on the coldest surfaces	Wet after showers, dries in hours	Fan airflow and run time
Splash	Tub edge, threshold, curtain hem, toilet base	Wet during use	Squeegee, door or curtain fit, caulk
Supply or drain leak	Hard-edged ring, swelling, wall-bottom peel	Damp through a dry spell	Plumbing first
Water behind tile	Hollow tile, cracked tub-to-floor joint	Never fully dries	Open and dry; see water behind tile
Toilet seal or tank sweat	Dark or soft floor at the base	Constant or seasonal	Seal or tank work

EPA's Brief Guide to Mold, Moisture and Your Home says recurring bathroom mold is usually prevented by running a fan or opening a window and cleaning more often. That holds for the first row only; ventilation will not touch a leak.

The exhaust code floor for a bathroom

The DOE Building America Solution Center fan guide gives the IRC 2021 and ASHRAE 62.2 requirement as 50 cfm intermittent or 20 cfm continuous, discharged outdoors at least 10 feet from a mechanical air inlet or 3 feet from a gravity inlet. ICC’s own reader refused our fetch, so the table quotes Washington’s adopted code as one primary example.

Requirement	What it says	Where written
Rate	50 cfm intermittent or 20 cfm continuous	Table M1505.4.4.1, WAC 51-51-1505
Delivered airflow	Rates mean airflow as installed; tested unless the duct meets the prescriptive table	M1505.4.4.2, WAC 51-51-1505
Controls	Manual on and off, or occupancy, humidity, timer or pollutant sensor	M1505.4.4.1, WAC 51-51-1505
Extra elbows	Minus 10 ft of allowed length per elbow beyond three	Table M1505.4.4.2, WAC 51-51-1505
Sound	ENERGY STAR: 1 sone continuous, 3 sones intermittent	DOE Building America Solution Center fan guide
Duct route	First 3 ft straight; an elbow at the outlet can significantly cut airflow	DOE Building America Solution Center fan guide

The duct table explains why a correctly rated fan can still fail. Per Washington’s table, a 50 cfm fan on 4 inch flex is good for 25 feet, against 70 feet of smooth duct, and an 80 cfm fan on 4 inch smooth only 20 feet.

Apartments, hotels and commercial restrooms

Space	Intermittent	Continuous	Per
Public toilet room	70 cfm	50 cfm	water closet or urinal
Hotel or dormitory private bath	50 cfm	25 cfm	room
Shower room	50 cfm	20 cfm	showerhead
Dwelling unit bathroom	50 cfm	20 cfm	bathroom

Sizing the fan: three methods, three answers

Code sets a floor, not a fit. The Home Ventilating Institute recommends 1 cfm per square foot up to 100 sq ft, about eight air changes an hour, and above that 50 cfm per toilet, shower and tub plus 100 cfm per jetted tub. Broan-NuTone's sizing help page uses volume: length times width times height times 0.13, rounded up to the next ten.

Both land near the eight air changes per hour that Shair and colleagues, ASHRAE Journal, 1979 suggested for bathroom moisture control: 0.13 cfm per cubic foot is 7.8 an hour, HVI’s rate in an 8 foot room 7.5. They part on ceiling height, which only Broan’s method counts.

Bathroom	Code floor	HVI	Broan
5 by 8 ft, 8 ft ceiling	50 cfm	40 cfm (floor governs)	50 cfm
10 by 8 ft, 9 ft ceiling	50 cfm	80 cfm	100 cfm
Over 100 sq ft: toilet, shower, tub	50 cfm	150 cfm	Not used
Same, jetted tub	50 cfm	200 cfm	Not used

HVI also says to undercut the door so make-up air can enter; a fan behind a tight door is starved.

How fast the steam actually leaves

Home Ventilating Institute says run the fan 20 minutes after use; the DOE Building America Solution Center fan guide says one hour. The chart runs the standard well-mixed dilution equation on the worked rooms: arithmetic, not a measurement.

Room and delivered airflow	Air changes per hour	Left at 20 min	Left at 60 min
5 by 8 ft hall bath, 50 cfm	9.4	4%	0%
Same room, only 25 cfm delivered	4.7	21%	1%
10 by 12 ft primary bath, 50 cfm	2.8	40%	6%
Same primary bath, 150 cfm	8.3	6%	0%

Per the model, a 5 by 8 ft bath with a working 50 cfm fan is nearly clear at HVI’s 20 minutes, about 4% left. A 10 by 12 ft bath on the same fan holds about 40% at 20 minutes and 6% at an hour, per the same arithmetic: big bathrooms with one small fan grow mold farthest from the grille.

Measured data agrees. In a 2022 SINTEF and NMBU master's thesis, the field bathroom at 36 cubic metres an hour, about 21 cfm, logged 15 minutes of mould-favourable conditions in a week; the one at 54, about 32 cfm, logged none.

Bathroom mold prevention: humidity targets and the meter test

EPA prices a humidity meter at about $10 to $50. Every bathroom spikes during a shower; what counts is the fall. Note the reading before the shower, run the fan, read again at 20 and 60 minutes. Still well above EPA’s 60% at the hour with the fan on and the door undercut? The fan, duct or termination is the problem. Hold a tissue to the grille first, as the DOE Building America Solution Center fan guide suggests.

How to get rid of mold in a bathroom, in room order

For area limits and protective gear, see DIY mold remediation. This is the sequence specific to this room.

1. Fix the air first. Why this differs: a bathroom refills with moisture daily, so cleaning first just resets the clock.
2. Rule out a leak. Why this differs: condensation and leaks sit side by side here, and cleaning over a leak hides the evidence a plumber or adjuster needs.
3. Clean top down. Ceiling, walls, fixtures, floor. Why this differs: the ceiling is often the largest patch; see mold on a bathroom ceiling.
4. Treat joints and soft goods separately. Why this differs: caulk, grout and curtains are often cheaper to replace than clean; see mold in shower and shower curtain mold.
5. Mind the chemistry. CDC's mold page says no more than 1 cup of household laundry bleach in 1 gallon of water for hard surfaces, never mixed with ammonia or other cleaners. Why this differs: a small closed room concentrates fumes.
6. Do not paint or caulk over it. EPA’s Brief Guide says paint over mold is likely to peel. Why this differs: mold-resistant bathroom paint is the usual shortcut.

Black mold in bathroom walls, and other signs it is bigger

Black mold low on a bathroom wall, beside the tub or behind the toilet, is usually the visible end of a wet assembly. These triggers mean open it up rather than clean again.

Trigger	Usually means	Next step
Back within weeks after the fan is proven	Water the fan cannot reach	Moisture readings, plumbing check
Hollow or cracked tile, cracked tub-to-floor joint	Water behind the tile	water behind tile
Soft floor at toilet or tub	Wet or decayed subfloor	Pull toilet or baseboard, probe
Peeling paint on the far side of a shower wall	Wet wall cavity	Open from the dry side
Stain on the ceiling below	Drain, supply or pan leak	Open the ceiling below first
Musty smell, every surface dry	Hidden growth	Inspection
Over about 10 square feet	EPA’s professional-help threshold	DIY mold remediation

CDC's mold page advises against carpet in bathrooms at all.

The bathroom decision path

1. Find the surface in the room map; take a dated photo.
2. Leave the room unused a day or two. Still damp: leak. Plumbing first, then step 7.
3. Dry between showers: hold a tissue to the grille. No pull means fan, duct or termination.
4. Pulls but recovers slowly: compare the fan to HVI sizing.
5. Sized and pulling: read the meter at 20 and 60 minutes; check duct length and the door gap.
6. Air fixed: clean each surface by its own page, top down.
7. Any escalation trigger: open the assembly or call a remediation contractor.

For contractors: scope, documentation and policy wording

ANSI/IICRC S520, Fourth Edition, 2024, sets mold remediation procedures. A bathroom job is small in area and heavy in documentation, because the cause decides who pays.

Record	What	Why the adjuster asks
Source	Readings against a dry reference; dry-spell result	Sudden leak and long-term seepage differ
Exhaust	Rating, duct size, length, elbows, termination	A failed fan is maintenance; an upgrade may be code work
Duration	Dated photos, stain layers, owner’s timeline	Wording turns on weeks, months or years
Concealment	Inside wall, under floor or above ceiling	Hidden and unknown damage is carved back in

The NJM New Jersey HO-3 (03/18) form, one published example, excludes constant or repeated seepage or leakage, or condensation of humidity, moisture or vapor, over weeks, months or years, unless unknown to all insureds and hidden within walls, ceilings or floors. The same form adds fungi, wet or dry rot, or bacteria coverage of up to $25,000, per the form’s wording including tear-out to reach growth. Forms differ; send owners to their policy, their state insurance department and a licensed professional.

Questions people actually ask

Is mold in the bathroom normal?

Small spots on grout, caulk or a ceiling corner are common because a bathroom gets wet daily. Per EPA’s Brief Guide, recurring bathroom mold usually means the room needs more ventilation and more frequent cleaning. Growth that returns after the fan is fixed, or sits low on a wall, points to a leak instead.

How do I get rid of mold in my bathroom permanently?

Remove the water, not just the stain. Prove the fan pulls with a tissue, size it to HVI’s 1 cfm per square foot, run it 20 to 60 minutes after showers, and rule out leaks with a dry-spell test. Then clean each surface by its own method, top down, and never paint over growth.

What size exhaust fan does a bathroom need?

The IRC floor, as summarised by DOE, is 50 cfm intermittent or 20 cfm continuous. HVI recommends 1 cfm per square foot up to 100 sq ft, and above that 50 cfm per toilet, shower or tub plus 100 cfm per jetted tub. Broan-NuTone multiplies room volume by 0.13.

How long should a bathroom fan run after a shower?

HVI recommends 20 minutes; DOE’s Building America Solution Center says one hour on a timer. On this page’s well-mixed model, 20 minutes clears a small bath with a working 50 cfm fan, but a large bath on the same fan still holds a big share of its moisture. Let a humidity meter decide.

Can a bathroom exhaust fan cause mold?

A fan venting into the attic or soffit, or a cold uninsulated duct, moves the moisture instead of removing it, and condensation can drain back to the grille. DOE’s guide and Washington’s adopted IRC require discharge outdoors. Mold on the grille usually means the duct needs checking from the attic.

What should I do about black mold in a bathroom wall?

If it sits on the paint and wipes off, clean it and fix the ventilation. If it is low beside the tub or toilet, returns within weeks, or comes with loose tile, peeling paint or a soft floor, treat it as a wet wall: find the water, check the cavity and remove wet drywall rather than cleaning it.