How to Find Mold

Have you ever walked into a room and smelled the signature musty scent of mold? But when you look around, there’s no mold in sight.  Or maybe you’ve started to experience headaches or allergy symptoms at work, but you can’t seem to figure out why. Both of these situations are pretty sure signs of mold growth. And just because you can’t find mold growth doesn’t mean you’re going crazy. It just means that mold is doing a good job of hiding itself.

Finding Mold

Hidden mold growth is incredibly common. Instead of growing out in the open, mold oftentimes grows underneath flooring, between walls and behind building materials.  Why? Because these locations provide ideal environments for mold growth with lots of trapped moisture and an endless food source. But it makes finding and identifying mold problems that much more difficult.

So how you do find mold that doesn’t want to be found? You look for other telltale signs that are difficult to hide.

Signs of Mold Growth

mold in corner

Mold tends to grow in corners of rooms and near the seams of wallpaper or carpet, as pictured here. Image: Thomas Anderson, CC 2.0

1. Mold or mildew sightings

Any visible mold or mildew sightings are (obviously) a sign of mold growth and could point to a larger mold or moisture problem. The reason we mention this is that any mold or mildew growth means that the building in question:

  • is hospitable enough for mold growth,
  • has active mold spores that can spread elsewhere, and
  • likely has either visible or hidden mold growth elsewhere.


powdery mildew can help you find mold

Powdery mildew on carpets or other building materials can be a sign of a greater mold problem. Image: Adam Kahtava, CC 2.0

Mold oftentimes appears as a stain or dark spots on the wall, or as stains creeping from the carpet or ceiling. Corners of rooms near the ceiling or floor are favorite growth spots,  as are window sills and walls near the ceiling or floor. You should also look for mold growth along seams of building materials, such as wallpaper seams, wall switch places, between flooring tiles and in tile grout.

Also keep your eyes peeled for mildew, which is is more difficult to see because of its light color. Mildew usually has a white fluffy texture, but it can also look like a white powdery substance on some building materials. So you if see white powder on the carpet, don’t just assume it’s dust! Read Mold vs. Mildew for more help distinguishing mold from mildew.

2. Musty smells & occupant symptoms

We mentioned that one of the biggest problems with mold growth is that it is not always visible. When hidden mold grows behind walls or under building materials we have to use our other senses to identify it. Luckily, mold has a tell-tale scent: a strong musty odor. Molds produce chemical gases called microbial Volatile Organic Compounds (mVOCs) during their growth cycle that give off this smell. Not all molds give off the same odor—but they are all characteristically musty.

MVOCs can also cause symptoms in building occupants, including headaches, nausea, dizziness, fatigue and increased allergy and asthma symptoms.  If occupants are experiencing any of these symptoms in combination with a musty odor in your building, it’s very likely that you have a hidden mold problem that needs to be investigated further.

3. Water

In our previous blog post, The Mold Equation, we learned that where there’s water, there can be mold growth. Buildings should have measures in place to control water, but sometimes those safeguards fail leading to water leaks, puddles and floods.

Any major water events or unexpected occurrences of moisture accumulation set the stage for mold growth. Water events might include leaky roofs, plumbing problems, puddles of standing water in basements, water seeping through windows and floods caused by inclement weather.

But water problems aren’t always so obvious. Excess moisture accumulation in general is also a problem. For example, standing water in HVAC drip pans can lead to mold growth if they are not drawing properly and cleaned regularly.

More commonly, however, you’ll see problems caused by excess moisture in the air. When indoor air is too humid, airborne moisture will condense on windows, walls, appliances, between walls, underneath flooring and everywhere in between. When condensation forms somewhere it cannot escape (like behind building materials or walls), you end up with the perfect habitat for hidden mold growth.

A lot of people assume that older buildings are at a higher risk for mold growth due to leaky roofs or plumbing problems, but condensation can lead to major mold growth even in brand new buildings. In fact, many newer building are more likely to experience hidden mold growth because they don’t allow for enough air movement through building materials. Ensure that your building’s HVAC system can properly manage indoor humidity levels to lower your risk for widespread hidden mold growth.

"Water Damage" by Ed! (Photography Gallery) - I created this work entirely by myself.. Licensed under CC BY-SA 3.0 via Wikipedia

Water stains, peeling wallpaper or paint, and other signs of water damage can help you find mold growth hidden behind walls and building materials.  ‘Water Damage’ by Ed! via Wikipedia, CC BY-SA 3.0

4. Damaged building materials

Mold growth almost always goes hand-in-hand with visible damage to building materials, including warping, rotting and discoloration. Damage can be caused by either mold growth itself, or by exposure to moisture related to mold growth. More information on how moisture affects building materials can be found here, but the following signs of building damage are almost always connected to mold:

  • Stained ceiling tiles
  • Bubbling behind paint or wallpaper
  • Dark stains on walls or at wallpaper seams
  • Discoloration on carpet backing


5. Other signs of mold growth can include:

  • Mold growth at wall switch plates
  • Powdery accumulation on carpet or other surfaces
  • Mold growth on exterior walls
  • Mold growth on materials like books and clothing

What should you do if you find mold?

So you think you’ve found mold. What should you do?

Here are the basic steps you should take to safely investigate and minimize the problem:

1. Report

First of all, let the appropriate person(s) know that you’ve either discovered or suspect mold. That might be your manager, facilities personnel, or the building owner.

In cases where mold growth is visible, it will be easy for facilities personnel to visually confirm your discovery. In other cases, you may only have a suspicion of mold growth. Don’t be afraid to share your observations with the appropriate personnel, as well as the reason(s) you suspect mold growth (be it a musty odor, mVOC-related health symptoms, or another visual clue); it’s likely that other occupants have noticed these problems as well.

Also, be sure to report any water leaks, puddles or unusual occurrences of water in your building. Even if there is not mold growth, these moisture incidents can lead to mold growth if they are not addressed.

2. Dry

In the case of a moisture incident, the key to preventing mold growth will be to dry building materials as quickly as possible. If materials are not dried within 48 hours the risk for mold growth increases substantially. Fans and increased ventilation can help speed up the process.

3. Investigate

If you think you’ve discovered a mold problem, you’ll have to confirm your findings. But that doesn’t mean it’s time to get your hands dirty. Instead, you want to limit interaction with the materials in question so that you don’t disturb the mold or put your health at risk. Instead, contact professionals to help you investigate the mold with a controlled survey and/or sampling. We’ll explore these options more in the Investigating Mold Growth section, below.

4. Clean & Remove Mold

This mold was hidden behind a poster that acted as a vapor barrier and trapped moisture behind it, leading to pretty extensive mold growth. Image: Angela Randall, CC 2.0

This mold was hidden behind a poster that acted as a vapor barrier and trapped moisture behind it, leading to pretty extensive mold growth. Image: Angela Randall, CC 2.0

Now for the not-so-fun part: remediation. That means cleaning affected materials until all mold is killed or removed. Unfortunately, mold tends to work its way deep inside building materials, so deep cleaning and chemicals won’t always kill the mold. This is especially common in moisture-sensitive products like wood, carpet, wallpaper and drywall; those products will need to be carefully removed and replaced.

As for who should do the clean-up—that depends on the situation. Cleaning up the mold yourself might be an okay option for a home project. But when it comes to other facilities where occupant health is at risk it’s essential that you consult with a professional remediation company and have them remove the mold. Schools, nursing homes, hospitals and other healthcare facilities will need to be especially strict about seeking professional help because children, the elderly and some patients are at increased risk for health effects from mold exposure.

We’ll go more in-depth on mold-clean up in the “Mold Clean-up” section, below.

5. Find & Fix

Once you’ve resolved a mold problem the last thing you want to do is deal with it again. Once the drama of mold clean-up is complete, make a point to determine the cause of the mold growth or moisture problem to prevent recurring mold growth. Leaky roof? Get it patched. Too much humidity in the air? Amp-up your HVAC system. Whatever the problem, make sure you fix it so you (hopefully) don’t have to deal with mold again!

6. Be Observant

Keep your eyes peeled for any signs of mold growth or moisture problems, particularly in areas where they’ve occurred before. If you’re vigilant, you can detect future mold and moisture problems early on before they become a major headache.

Investigating Mold Growth

Investigating hidden mold problems may be difficult and will require caution, especially when the investigation involves disturbing potential sites of mold growth.

While you should be able to confirm visible mold with a simple visual inspection, investigating the presence of hidden mold is much trickier. Before you start knocking down walls, explore your sampling options. This could save you big bucks and will prevent the disturbance of mold spores, which is key to protecting occupant health and preventing future mold growth.

Sampling for mold

There are several types of samples that can be used to determine or confirm the presence of mold. Here are some of the most popular options:

Air Samples

Air samples are great for testing mold spore concentrations in sensitive environments like hospitals where the presence of any mold spores is risky. But it’s not always the best for verifying mold growth in general. Mold spores are present in pretty much all indoor and outdoor air, so the presence of airborne mold spores is not an issue: it’s actual mold growth that’s the problem.

In the case of visible mold growth, air sampling will be pretty unhelpful. However, if you’re investigating hidden mold growth, a high concentration of airborne spores could help confirm suspicions of mold and identify the type of mold before you start tearing up building materials. Air samples can also be used at the conclusion of a project to confirm that all mold growth has been uncovered and remediated.

Bulk Samples

Bulk samples are taken by removing part of a mold-contaminated material (with the mold intact), sealing the sample and sending if off to a lab for testing. The lab will analyze the mold spores, confirm or deny the presence of mold on the material, and possibly even identify which genus the mold belongs to.

Surface Samples

Surface samples involve carefully removing suspect mold from a surface, sealing the sample and sending it to a lab. Cotton swab samples can be can be used to confirm or deny the presence of mold and are oftentimes used in follow-up testing following mold remediation. Tape lift samples can be used to determine the concentration of mold spores on a given surface.


Investigating Mold in At-Risk Environments

If the mold growth you are investigating is in a school, long-term care facility, hospital or other healthcare facility, you will need to contact a professional to conduct any investigations of suspect mold. They will install the appropriate containment measures and use sampling techniques that prevent building occupants from being exposed to potentially harmful mold spores.

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