How to Find and Control Mold in a Home or Business

You may be wondering whether your home or business has a mold problem. If you already know it does, you’ll need to know how to get rid of the mold and keep it from coming back. Mold thrives in moist, warm environments, so wet or humid areas are prime locations.

In this article we’ll cover a number of topics, including:

• What Is Mold?
• Many Sources of Moisture Intrusion in the Home
• Molds: the Good, the Bad, and the Ugly
• Assessing a Mold Problem
• How to Find Moisture Behind Walls
• Other Necessary Tests
• Mold Remediation

Besides the health risks associated with some molds, the moist conditions mold needs can cause bigger structural problems. Too much moisture in a home or building can make wood swell, warp, or even rot, weakening the structure. According to the U.S. Environmental Protection Agency’s guide to mold and moisture, “the key to mold control is moisture control.”

What Is Mold?

Molds are fungi that grow everywhere, both indoors and outdoors. They grow best in warm, damp, humid conditions. When conditions are right, mold can grow even in dry climates such as Arizona.

Molds reproduce by forming spores that spread through the air. Because mold spores are everywhere, there’s no practical way to eliminate them from indoor environments or create a mold-free space. We can, however, limit mold growth by controlling moisture indoors.

Mold needs four things to grow:

• Mold spores
• Nutrients: any organic material, such as wood, paper, or drywall
• Moisture: water, condensation, or damp air (the EPA recommends keeping indoor relative humidity below 60%)
• Suitable temperatures: most molds grow well at the same temperatures people find comfortable

There are many thousands of mold species, and many different types can be found indoors. Only some are known to cause illness.

Common types of mold found indoors include:

• Aspergillus species (such as A. flavus and A. versicolor)
• Cladosporium
• Penicillium
• Alternaria
• Stachybotrys chartarum (also called S. atra), often called “black mold”

Mold often grows where you can’t easily see it: behind wallpaper or paneling, on the top side of ceiling tiles, on the back side of drywall, or under carpet, carpet padding, or wood flooring.

Pipes inside walls can also lead to mold growth, since leaks and condensation add moisture. Roofs sometimes leak too, letting water collect inside walls and insulation.

Many Sources of Moisture Intrusion in the Home

Indoor water leaks, broken pipes, a clogged air conditioner drain line, a leaky roof, poorly sealed windows, basement flooding, and steam from showers or cooking are common ways moisture becomes a problem in homes, apartments, schools, and workplaces.

Outside Sources

Many other conditions can let water in and lead to mold growth. For instance, gutter downspouts or lawn sprinklers placed too close to an outside wall can send water straight into a basement or saturate a concrete foundation. Once the concrete is wet, moisture can wick into the building.

Construction Issues

Construction problems can also cause mold growth: plumbing punctured by nails or screws during construction, improperly sealed bathtub drains, sinks, and garbage disposals, or particleboard that was installed after getting rained on during construction.

Water Vapor

When indoor relative humidity rises to 60% or more, building materials and furnishings absorb the moisture, and those damp materials give mold a place to grow. If relative humidity stays above 60% for an extended period, mold growth is likely.

Seasonal Changes

Warm air can hold more moisture than cool air, which can cause moisture problems. Consider what happens in fall or winter. When warm indoor air touches a cold surface, such as a single-pane window or an uninsulated wall, the air cools and the excess moisture condenses. That moisture can lead to mold and mildew.

Can mold grow on concrete?

Wet or damp concrete slabs often contribute to indoor mold problems. Concrete absorbs water, and that by itself isn’t the problem. But materials attached to wet concrete, such as wood flooring, carpet, cabinets, and wood framing, can absorb moisture from the slab and become ideal food sources for mold.

Concrete can get wet in several ways. Without a good vapor retarder under a slab, moisture can wick up into the concrete from ground that’s damp because of irrigation or poor drainage.

This can be a problem where the ground slopes toward a building, sending rain and irrigation water to the slab until it becomes saturated.

In some cases, the concrete never finished drying after it was poured. Contractors under pressure to meet deadlines may not allow enough drying time before construction continues, and the slab can stay wet for months or even years longer than it should.

Wherever there’s a water or moisture problem, a mold problem often follows quickly.

“Molds can cover large areas within 24 to 72 hours after water damage occurs,” says Dr. Luke Curtis, a medical doctor and Certified Industrial Hygienist who has assessed more than 1,000 buildings for mold, moisture, and other indoor air quality problems. The EPA likewise advises drying water-damaged areas and items within 24 to 48 hours to prevent mold growth.

Molds: the Good, the Bad, and the Ugly

Believe it or not, many molds are beneficial. They help decompose dead material, they’re the source of antibiotics such as penicillin, and they’re used to make cheeses. Some are also used to produce enzymes commercially.

But many molds are harmful. They grow on bread and other foods, damage grain, fruit, vegetables, and livestock feed, and cause diseases in garden plants. Related fungi even cause athlete’s foot and ringworm.

Mold spores, whether dead or alive, can also cause health problems in people, especially those who are sensitive to mold. Symptoms can include nasal stuffiness, eye irritation, wheezing, and skin irritation.

Assessing a Mold Problem

If there’s only minor surface mold, a home or building owner usually doesn’t need an expert. But if the mold is widespread, has caused visible damage, or is suspected of causing health problems, it’s best to call a certified professional trained in dealing with mold.

Professionals trained in mold assessment and remediation are often certified through organizations such as NORMI (National Organization of Remediators and Mold Inspectors) or the IICRC (Institute of Inspection, Cleaning and Restoration Certification).

Certified Industrial Hygienists (CIHs) also assess mold problems. CIHs are certified by the Board for Global EHS Credentialing (formerly the American Board of Industrial Hygiene), and many belong to the American Industrial Hygiene Association (AIHA).

It’s important to hire someone trained in mold identification and remediation. Mold assessment can easily be done wrong by untrained people. After Hurricanes Sandy, Irene, Rita, and Katrina, for instance, many companies sprang up claiming to understand mold contamination, failed to follow proper protocols, and cost many home and business owners both property and money.

Ron Guncheon, owner of RMS Environmental in North Palm Beach, Florida, and a certified member of NORMI, explains that a mold specialist can approach a job in one of two ways: mold assessment or mold remediation. In Florida, state law doesn’t allow the same professional to do both on a property, because that would be a conflict of interest.

A mold assessment determines whether mold is present, through a visual inspection and testing.

When Guncheon does an assessment, he starts by asking his clients about any health issues, such as a runny nose, trouble breathing, nasal stuffiness, or other respiratory symptoms.

He also asks whether their symptoms occur only at home or elsewhere too, and whether they show up in one particular room.

“I’m looking for signs of reactions people will have when they get exposed to mold and mold spores. After that, I conduct a visual inspection indoors and outdoors.”

He looks for visible signs and odors of mold. That may mean checking baseboards, under carpets, air conditioners, ductwork, under sink cabinets, showers, and even outside walls. If he can’t smell or see any signs of mold, he uses an infrared camera to look for hidden moisture.

How to Find Moisture Behind Walls

Moisture behind walls or under floors can often be detected with a sensitive infrared camera, which produces a temperature map of the surface. Damp areas often show up differently from the surrounding area because wet materials are usually cooler than dry ones.

Infrared inspection is a fast, noninvasive way to find possible moisture problems in a home or building, but it doesn’t detect mold itself. Instead, it points to damp spots where mold may develop.

“Once I find an area where there is moisture or water vapor, I use a moisture meter to tell me the level of moisture present. If the level is too high, there’s a good chance mold is growing as a result. I then take a picture of the meter’s reading to document my findings,” Guncheon says.

Wagner Meters’ Orion^® 950 smart pinless meter makes documenting moisture levels easy. It uses electromagnetic technology to measure moisture without damaging the material. Besides reading the moisture content of wood, it has a relative measurement mode (a 0 to 100 scale) for non-wood building materials such as drywall.

For inspection work, that relative reading gives you a comparison. Take a reading on a known dry area of the same material to set a baseline, then compare readings to pinpoint areas with elevated moisture.

Other Necessary Tests

Guncheon also checks indoor temperature and relative humidity throughout the building, looking for unusual differences between rooms. The EPA recommends keeping indoor relative humidity below 60%, and ideally between 30% and 50%.

For hard-to-reach areas, such as cavities in ceilings, walls, and floors, Dr. Curtis sometimes uses fiber optic devices. Some carry light into confined spaces, while others have video capability so the inspector can see areas that are hard to access.

“A number of investigators even use dogs trained to sniff mold. I, myself, once used a mold-sniffing dog to find heavy mold growth in a hard-to-reach ventilation duct,” he says.

Whether or not mold is found, Guncheon takes samples and sends them to a qualified lab, which cultures and identifies any mold species present. He uses three to five sampling methods, depending on what the client wants to pay for:

• Air sampling: a device draws in air for 5 minutes to collect whatever is floating in it, and the sample goes to a lab.
• Petri dish: material is placed in a Petri dish, which is sent to a lab.
• Tape lift: mold is collected on sticky tape and placed in a sealed bag that’s sent to a lab.
• Bulk: pieces of the contaminated material are removed and sent to a lab.
• Swab test: a cotton swab is rubbed against a surface, placed in a test tube, and sent to a lab for mold identification.

If he can’t find moisture or water intrusion or locate mold growth, or if the client had earlier mold testing that found no mold but they’re still having problems, he runs a VOC (volatile organic compounds) test. Molds and many other things in a building give off VOCs, and this test helps identify them.

The VOC sampler runs for 2 hours to collect air samples, which are sent to a lab for analysis. The lab reports what compounds are present and whether their levels are acceptable, moderate, or excessive.

Guncheon presents his findings in a report to the client. It typically identifies the problem, locates the mold, determines the cause (for example, a broken pipe), identifies the mold species, and describes what needs to be done. The fix may be as simple as cleaning and treating the area, or it may require moderate to extensive mold remediation.

Mold Remediation

Mold remediation is simply the professional term for mold removal. If remediation is needed, the assessment details what’s required.

It’s up to the client to follow through on the assessment. They can do some of the work themselves (for example, tearing out a wall) or hire a mold remediation specialist to handle everything and keep contamination to a minimum.

The client also has to decide who does the build back. For instance, if a wall is opened up, the client decides whether the remediation specialist repairs it or someone else does.

Guncheon says that if he finds an infested wall, say in a bedroom, he may recommend walling off the whole area from the rest of the house. He builds a containment wall of 6-mil plastic with a zipper entrance, held in place with tape and supports and attached to the walls, floor, and ceiling, so the area won’t contaminate the rest of the house.

“I use negative air pressure to exhaust to the outside whatever mold spores and contaminants are present in the room. (This is similar to turning on a bathroom vent fan to remove steam.) If you have water in the room, it has to be dried out first before proceeding,” he says.

Inside the containment area, he uses air scrubbers with HEPA (high-efficiency particulate air) filters to trap mold spores and other particles before the air is exhausted outdoors. That creates negative air pressure, so air from the rest of the house is drawn into the work area and then exhausted outside. “If we don’t do that, then we’re allowing the spores to roam freely through the rest of the house,” he adds.

Wearing protective gear, such as a respirator, goggles, protective gloves, and a disposable protective suit, Guncheon begins taking the affected area apart. Removed materials, such as drywall or damaged wood, are carefully bagged.

Once the affected pieces are bagged, the area is carefully HEPA vacuumed to capture spores.

Remaining mold growth is carefully removed, and the area is cleaned and treated. In some cases, he may fog the entire house with an antimicrobial agent.

Simply killing mold isn’t enough. According to the EPA, using a biocide such as chlorine bleach is not recommended as a routine practice during mold cleanup, and dead mold can still cause allergic reactions in some people, so it has to be removed, not just killed. The EPA also warns never to mix chlorine bleach with cleaning products that contain ammonia, because doing so produces toxic fumes. And unless the moisture problem is fixed, the mold will grow back.

Build Back

Once the mold is removed, the source of moisture (for example, a broken water pipe) is corrected, and the damaged materials are removed, the final step is the build back. The client can have the mold remediation specialist do the repairs, hire someone else, or do the repairs themselves.

In Closing

Mold is everywhere. There’s no escaping it, but we can keep it from becoming a problem by controlling moisture and excess humidity indoors.

When mold does appear, it can damage property and cause health problems. That’s when it’s best to hire a trained, certified mold specialist with the knowledge and tools to find, identify, and eliminate the problem.

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Written by Wagner Meters and first published as a three-part series in ICS Cleaning Specialist magazine (August, September, and October 2014).

Last updated on October 8th, 2026