National Home Inspection Service



Mold Basics

Man shocked to mold a kitchen cabinet.
Mold Basics

Mold Basics

  • The key to mold control is moisture control.

  • If mold is a problem in your home, you should clean up the mold promptly and fix the water problem.

  • Drying water-damaged areas and items must be dry within 24 to 48 hours to prevent mold growth.


 Why is mold growing in my home?

Molds are part of the natural environment.  Outdoors and molds play a part in nature by breaking down dead organic matter, such as fallen leaves and dead trees.  But indoors, mold growth should be avoided.  Molds reproduce utilizing tiny spores; the spores are invisible to the naked eye and float through outdoor and indoor air.  Mold may begin growing indoors when mold spores land on wet surfaces.  There are many types of mold, and none of them will grow without water or moisture.


Can mold cause health problems?

Molds are usually not a problem indoors unless mold spores land on a wet spot and begin growing.  Molds have the potential to cause health problems.  Molds produce allergens (substances that can cause allergic reactions), irritants, and, in some cases, potentially toxic substances (mycotoxins).  Inhaling or touching mold or mold spores may cause allergic reactions in sensitive individuals.  Allergic responses include hay fever-type symptoms, such as sneezing, runny nose, red eyes, and skin rash (dermatitis).  Allergic reactions to mold are common.  They can be immediate or delayed.  Molds can also cause asthma attacks in people with asthma who are allergic to mold.  In addition, mold exposure can irritate mold-allergic and non-allergic people’s eyes, skin, nose, throat, and lungs.  Symptoms other than the allergic and irritant types are not commonly reported due to inhaling mold.  Research on mold and its health effects is ongoing.  This article provides a brief overview; it does not describe all potential health effects of mold exposure.  For more detailed information, consult a health professional.  You may also wish to consult your state or local health department.

How do I get rid of mold? 

It is impossible to get rid of all mold and mold spores indoors.  Some mold spores will float through the air and in house dust.  Mold spores will not grow if moisture is not present.  Indoor mold growth can and should be controlled by controlling moisture indoors.  If mold grows in your home, you must clean up the mold and fix the water problem.  If you clean up the mold but don’t fix the water problem, then, most likely, the mold problem will recur.


Who should do the cleanup?

This depends on several factors.  One consideration is the size of the mold problem.  If the moldy area is less than about 10 square feet (less than roughly a 3-foot by 3-foot patch), in most cases, you can handle the job yourself, following the guidelines below.

  • If there has been a lot of water damage and mold growth covering more than 10 square feet, consult with an InterNACHI inspector.

  • If you hire a contractor (or other professional service provider) to do the cleanup, ensure the contractor has experience cleaning up mold.  Check references and ask the contractor to follow the recommendations of the EPA, the American Conference of Governmental Industrial Hygienists (ACGIH) guidelines, or other guidelines from professional or government organizations.

  • Do not run the HVAC system if you know or suspect that it is contaminated with mold.  This could spread mold throughout the building.

  • If the water and mold damage was caused by sewage or other contaminated water, call in a professional with experience cleaning and fixing buildings damaged by contaminated water.

  • If you have health concerns, consult a health professional before starting cleanup.


Tips and Techniques

The tips and techniques presented in this section will help you clean up your mold problem.  Professional cleaners or remediators may use methods not covered here.  Please note that mold may cause staining and cosmetic damage.  It may not be possible to clean an item so that its original appearance is restored.   

  • Fix plumbing leaks and other water problems as soon as possible.  Dry all items completely. Scrub mold off hard surfaces with detergent and water, and dry thoroughly.

  • Absorbent or porous materials, such as ceiling tiles and carpet, may have to be thrown away if they become moldy.  Mold can grow on or fill in the empty spaces and crevices of porous materials, making the mold difficult or impossible to remove.

  • Avoid exposing yourself or others to mold.

  • Do not paint or caulk moldy surfaces.

  • Clean up the mold and dry the surfaces before painting.  Paint applied over moldy surfaces is likely to peel.  If you are unsure how to clean an item or if the item is expensive or of sentimental value, you may wish to consult a specialist.  Specialists in furniture repair and restoration, painting and art restoration and conservation, carpet and rug cleaning, water damage, and fire or water restoration are commonly listed in phone books.  Be sure to ask for and check references.  Look for specialists who are affiliated with professional organizations.


What to Wear When Cleaning Moldy Areas:  

  • Avoid breathing in mold or mold spores.  To limit exposure to airborne mold, you may want to wear an N-95 respirator, which is available at many hardware stores and companies that advertise on the Internet.  (They cost about $12 to $25.)  Some N-95 respirators resemble a paper dust mask with a nozzle on the front, and others are made primarily of plastic or rubber and have removable cartridges that trap and prevent most of the mold spores from entering.  To be effective, the respirator or mask must fit properly, so carefully follow the instructions supplied with the respirator.  Please note that the Occupational Safety and Health Administration (OSHA) requires that respirators fit properly (via fit testing) when used in an occupational setting.

  • Wear gloves.  Long gloves that extend to the middle of the forearm are recommended.  Ordinary household rubber gloves may be used when working with water and a mild detergent.  If you use a disinfectant, a biocide such as chlorine bleach, or a strong cleaning solution, select gloves made from natural rubber, neoprene, nitrile, polyurethane, or PVC.  Avoid touching mold or moldy items with your bare hands.

  • Wear goggles.  Goggles that do not have ventilation holes are recommended.  Avoid getting mold or mold spores in your eyes.


How do I know when the remediation or cleanup is finished?

You must have completely fixed the water or moisture problem before the cleanup or remediation can be considered finished, based on the following guidelines:

  • You should have completed the mold removal.  Visible mold and moldy odors should not be present.  Please note that mold may cause staining and cosmetic damage.

  • You should have revisited the site(s) shortly after cleanup, and it should show no signs of water damage or mold growth.

  • People should have been able to occupy or re-occupy the area without health complaints or physical symptoms.

  • Ultimately, this is a judgment call; there is no easy answer.  Ask your InterNACHI inspector during your next scheduled inspection if you have concerns or questions.

Moisture and Mold Prevention and Control Tips

  • Moisture control is the key to mold control, so when water leaks or spills occur indoors, ACT QUICKLY.  If wet materials or areas are dried within 24 to 48 hours after a leak or spill happens, mold will not grow in most cases.

  • Clean and repair roof gutters regularly.

  • Ensure the ground slopes away from the building’s foundation so that water does not enter or collect around the foundation.

  • Keep air-conditioning drip pans clean and the drain lines unobstructed and flowing properly.

  • Keep indoor humidity low.  Keep indoor humidity below 60% relative humidity (ideally, between 30% to 50%) if possible.  Relative humidity can be measured with a moisture or humidity meter, a small, inexpensive instrument (from $10 to $50) available at many hardware stores.

  • If condensation or moisture is collected on windows, walls, or pipes, ACT QUICKLY to dry the wet surface and reduce the moisture/water source.  Condensation can be a sign of high humidity.

 Actions that will help to reduce humidity:
  • Vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters, to the outdoors, where possible.  (Combustion appliances, such as stoves and kerosene heaters, produce water vapor and will increase the humidity unless vented to the outside.)

  • Use air conditioners and de-humidifiers when needed.

  • Run the bathroom fan or open the window when showering.  Use exhaust fans or open windows when cooking, running the dishwasher, washing, etc.

 Actions that will help prevent condensation:
  • Reduce the humidity (see above).

  • Increase ventilation and air movement by opening doors and windows when practical.  Use fans as needed.

  • Cover cold surfaces, such as cold water pipes, with insulation.

  • Increase air temperature.


Testing or Sampling for Mold

Is sampling for mold needed?  In most cases, sampling is unnecessary if visible mold growth is present.  Since no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building’s compliance with federal mold standards.  Surface sampling may be helpful to determine if an area has been adequately cleaned or remediated.  Sampling for mold should be conducted by professionals who have specific experience in designing mold sampling protocols, sampling methods, and interpreting results.  Sample analysis should follow analytical methods recommended by the American Industrial Hygiene Association (AIHA), the American Conference of Governmental Industrial Hygienists (ACGIH), or other professional organizations.

Suspicion of Hidden Mold

You may suspect hidden mold if a building smells moldy but cannot see the source, or if you know there has been water damage, residents report health problems.  Mold may be hidden in places such as the backside of drywall, wallpaper, paneling, the top side of ceiling tiles, or the underside of carpets and pads.  Other possible locations of hidden mold include areas inside walls around pipes (with leaking or condensing pipes), the surface of walls behind furniture (where condensation forms), inside ductwork, and in roof materials above ceiling tiles (due to roof leaks or insufficient insulation).

Investigating Hidden Mold Problems

Investigating hidden mold problems may be difficult and will require caution when the investigation involves disturbing potential sites of mold growth.  For example, wallpaper removal can lead to a massive release of spores if mold grows on the underside of the paper.  If you believe you may have a hidden mold problem, consider hiring an experienced professional.

Cleanup and Biocides

Biocides are substances that can destroy living organisms.  Using a chemical or biocide that kills organisms such as mold (chlorine bleach, for example) is not recommended as a routine practice during mold cleanup.  There may be instances, however, when professional judgment may indicate its use (for example, when immune-compromised individuals are present).  In most cases, it is not possible or desirable to sterilize an area; a background level of mold spores will remain, and these spores will not grow if the moisture problem has been resolved.  If you choose to use disinfectants or biocides, always ventilate the area and exhaust the air to the outdoors.  Never mix chlorine bleach with other cleaning solutions or detergents that contain ammonia because toxic fumes could be produced.   

Please note: Dead mold may still cause allergic reactions in some people, so it is not enough to kill it; it must also be removed.

Ten Things You Should Know About Mold

1.  Potential health effects and symptoms of mold exposure include allergic reactions, asthma, and other respiratory complaints.

2.  There is no practical way to eliminate all mold and mold spores in the indoor environment; the way to control indoor mold growth is to control moisture.

3.  If mold is a problem in your home, you must clean up the mold and eliminate sources of moisture.

4.  Fix the source of the water problem or leak to prevent mold growth.

5.  Reduce indoor humidity (to 30% to 60%) to decrease mold growth by:

  • venting bathrooms, dryers, and other moisture-generating sources to the outside
  • using air conditioners and de-humidifiers;
  • increasing ventilation and
  • using exhaust fans whenever cooking, dishwashing, and cleaning.

 6.  Clean and dry any damp building materials and furnishings within 24 to 48 hours to prevent mold growth.

7.  Clean mold off hard surfaces with water and detergent and dry thoroughly.  Absorbent moldy materials (such as carpeting and ceiling tiles) may need to be replaced.

8.  Prevent condensation.  Adding insulation reduces the potential for condensation on cold surfaces (i.e., windows, piping, exterior walls, roof, and floors).

9. Do not install carpeting in areas with a perpetual moisture problem.


10.  Molds can be found almost anywhere; they can grow on virtually any substance, provided moisture is present.  Some molds can grow on wood, paper, carpet, and food.






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