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Mold in bathroom

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Bathroom Mold - General Information

Showers, toilets and humidity are common causes of mold and moisture buildup in bathrooms.  Problems can run the gamut from minor surface mold in the shower to major wood rot and structural repairs.  

images of bathroom mold

Is bathroom mold a health problem?

Mold growth in the bathroom is less likely to cause health problems than other portions of the home.  This is due to a couple factors, primarily the limited amount of time occupants spend in the room, and additionally, many bathroom mold problems occur behind sealed surfaces.

For example, if a shower supply line cracks and causes systemic mold growth and wood rot in the wall cavity, the air quality in the bathroom is unaffected. This is because the growth is occurring in a sealed environment within the wall cavity.  

Where is bathroom mold growth typically found?

  • Bathroom Ceiling.  When a shower or bathtub is used, the relative humidity in the bathroom rockets to 100%, soon depositing moisture on the exposed surfaces.  Because warm air rises, the ceiling is often the first surface to experience condensation.   This is why mold growth is typically noted on bathroom ceilings long before it accumulates on the walls.  
black bathroom ceiling mold


  • Shower Mold.  Showers can fail in 3 ways: supply line, waste line or a leaky liner.  Failed supply or waste lines will often cause substantial structural damage and rot before they are detected.  The shower surround itself however, should exhibit outward signs of deterioration before major damage occurs.  The most likely failure points are grout between tiles, caulking at the lower edge and along fixtures and cracks within the tiles themselves.  In my experience, it is surprising how much moisture can migrate through a small crack in exposed grout.
shower mold image


  • Bathroom Exhaust Fan Without a properly working bathroom exhaust fan, mold growth is nearly inevitable.  The phrase 'properly working' needs some explanation.  Just because a bathroom fan makes a lot of noise, doesn't mean it's working properly.  In fact, the best bathroom fans operate almost undetectably.  An average size bathroom should have a minimum of 80CFM under full power.  Newer models from Panasonic offer continuous flow at 30-40 CFM and ramp up to full speed when the room is occupied.  Mold inspectors carry equipment capable of measuring the effectiveness of your bath fan, however the best indication of an inadequate fan is simply excessive moisture. 
bathroom mold - exhaust fan


  • Toilet Moisture & Mold. While not often a health concern, failed toilet seals can cause significant wood rot.  Look for signs of staining on the adjacent flooring.  Also, feel for sponginess or undue flexing in the subfloor surrounding the toilet. If the toilet is located on the ground floor, check the crawl space for signs of leakage.  Occasionally, toilets will suffer from condensation on the back and bottom side of the tank.  This can lead to mold growth on the wall immediately behind the toilet.
bathroom mold - toilet


  • Shower Tile Mold & Mildew.  Nearly every home will experience mold & mildew in the shower at some point.  Thankfully, mold growth on the bathroom shower tiles does not typically indicate a major health or structural problem.  We've conducted many air quality tests within bathrooms suffering from shower tile mold and never found an effect on the IAQ.  Prevention, of course, is the best remedy.  I've found 3 methods that dramatically reduce mold growth on shower tiles.  Squeegee after each shower use, run the bath fan for 1 hour and properly seal the grout.
bathroom mold - shower tile



Recent Bathroom Mold Inspections

Property Type:  Single Family Residence

Reason for inspection:

  • Client is experiencing significant allergy problems, primarily when sleeping inside his room.  Client feels better when sleeping out in the living room. No apparent source of contaminates.

 Summary of concerns:

  • Excessively high relative humidity levels inside the home.
  • Poor ventilation thorughout the home.
  • Elevated moisture levels noted in flooring in front of rear french doors.
  • No ventilation fan in bathroom.
  • Visible mold growth noted on bathroom walls due to excessive condensation.
  • Elevated moisture levels noted in flooring around toilet / in front of tub.
  • Water damage noted around window frame inside shower surround. 

LOCATION: Hall Bathroom

Summary:  Abnormal Conditions Found!


This bathroom may win the record for the number of simultaneous mold problems.  During the inspection we found inadequate ventilation (missing exhaust fan), visible mold growth on wall & ceiling, water damage on the window trim, mold & water damage from missing grout in the back splash, elevated moisture in the subfloor and lastly, water pooling in the crawl space from plumbing leak.

bathoom mold cause by a missing fan

No bathroom fan = mold problems.


The bathroom does not have any exhaust ventilation currently installed.  A window is present, but this is inadequate for proper moisture removal.  Bathroom windows are effective at reducing your energy efficiency and not much more.  Additionally, even if a bathroom window was functioning well at the removal of moisture, most people lack the discipline to open a window in the middle of winter.  This, of course, is when the fresh air is needed the most.  We typically recommend Panasonic WhisperGreen fans.  They operate continuously at a low CFM and ramp up to full speed automatically when the bathroom is occupied. 

RH: 68%            Temp: 67°F            CO2: 974ppm           CO: 0ppm

Bathroom Mold Growth

Bathroom mold growth on wall

The lack of proper ventilation doesn't always lead to visible mold growth.  Many factors effect the actual actual growth of the mold: quality of the paint surface, level of insulation, etc.  However, this bathroom clearly suffers from visible mold growth on the upper walls and ceiling.  Thankfully, this type of mold growth is condensation based, which often can be cleaned and removed without removing the sheetrock itself.  This is due to the fact that airborne moisture (which occurs when an exhaust fan is missing) causes mold growth only on the side facing the humid conditions.  For example, if we were to look at the backside of these moldy walls, they would undoubtedly have no mold growth whatsoever.  Additionally, this type of bathroom mold growth typically does not cause elevated mold spore levels throughout the home. 

Dampness in Bathroom Floor

Bathroom Mold due to moisture under tile

Moisture meter testing indicated elevated dampness in the flooring materials adjacent to the bathtub and toilet. Usually this is due to a failure of the wax ring on the toilet or water pooling from shower usage.  In this case, the area of dampness is directly between the two potential culprits, making a final assessment difficult.  Often the cause can be differentiated by inspecting the area from the craw space.  However in this case, the crawlspace didn't yield any obvious clues.  

Dampness in the bathroom floor is more of a structural issue than an air quality problem.  Because the flooring does not allow air flow to pass through, mold spores cannot migrate into the conditioned air space.  

Bathroom Shower Windows?

Anytime you mix heavy water exposure and manufactured wood products you're asking for trouble.  As the photo indicates, moisture exposure has lead to cracked joints, mold growth and swelling.  Though not likely to cause an indoor air quality issue, these construction defects can lead to expense wood rot within the wall cavity.  If you insist on having a window within the shower enclosure, case it completely in impervious materials such as tile, granite, etc.  

Bathroom window mold & moisture problems

Vanity Problems.

Though it is difficult to determine from the photograph, the junction of the back splash and the tile counter is missing grout or sealant.  This typically isn't a major issue unless the counter is sloped to the rear.  In this case however, the moisture has lead to mold growth on the sheet rock wall behind the vanity.  

Bathroom Mold growth beneath vanity

Laboratory Results:

Though the samples were not collected from the bathroom, they are an indication of the overall air quality throughout the house.  In this case, the mold and IAQ problems were not confined merely to the bathroom. Moisture issues and highly elevated RH levels were found throughout the home.  

Bathroom mold testing results


  • Clean bathroom wall with a mild solution of soap and water and seal the surfaces with a high quality paint and primer.
  • Make necessary repairs around the window in the bathroom to ensure water does not enter the wall cavity.
  • Hire a qualified plumber determine the source and make necessary repairs to the leak noted in crawlspace.
  • Install a Panasonic Whisper Green® ventilation fan in bathroom to assist in evacuating the relative humidity from inside the building.  Whisper Green ventilation fans operate consistently at a low volume (30 cfm) and automatically ramp up to a higher volume (80 cfm) when a motion sensor is activated. An integrated timer will maintain the higher volume for a set period of time and then return the fan to the lower operating level until activated again.
  • Install a bead of silicone sealant along the edge of the backsplash to prevent moisture wicking below. 
  • Repair and reseal all damaged grout.