Mold Growth on Concrete - How to Clean it and How to Prevent Regrowth
First, is it really mold growth?
Identifying mold growth on concrete can be a bit tricky. If it’s black mold growth, it is easy to identify. But if it’s white mold growth, things can get confusing. There are several similar looking substances that can fool you.
Efflorescence can look very similar to white mold growth. It occurs when moisture moves through concrete (or other types of masonry) and pulls minerals along with the water. As the water evaporates at the surface, the salts and minerals are left behind, leaving a crystalline growth.
Differentiating between mold and minerals is easy. Spray a small amount of water on the substance. If it dissolves away, it’s efflorescence. If it doesn’t, it’s likely mold growing in your basement.
Why does mold grow on concrete?
Moisture. This is the one variable we can control when dealing with mold. With concrete you have 3 primary avenues for excess moisture.
Liquid water intrusion
If any part of your home is below grade, you’ve essentially created a giant empty bowl surrounded by water. Without proper drainage and/or a sump pump, the water will eventually make it through the concrete. Large cracks or holes are not necessary. Hydrostatic pressure is quite powerful and can force large amounts of water through hairlines cracks. Liquid water intrusions certainly can cause mold growth, however it is usually quite localized.
Humidity based mold growth
Concrete is a poor insulator. During the winter months when the temperature drops, the cool surface of a concrete wall can cause condensation. In time this elevated surface moisture creates an environment conducive to mold growth.
Mold growth on concrete due to humidity is typically diffuse and spread across a large area.
On it’s own, concrete does not provide a sufficient food source for fungal growth. Unfortunately this is not especially helpful, because like almost anywhere in a building, dust quickly accumulates on the surface. Within this layer of dust large quantities of food sources for mold are present. Therefore, unless the concrete is kept extremely clean, mold growth can occur.
Of course, even if the concrete is covered in dust, mold will not grow without sufficient moisture. In most situations, controlling the moisture, rather than the dust, is far more desirable. This includes tackling both airborne moisture (lowering the humidity) and liquid moisture (flooding, vapor drive).
How do I remove mold from concrete?
Thankfully, because of the extreme durability of concrete, removing mold growth is a relatively simple process. Unlike sheet rock, framing or carpeting, you don’t need to worry about damaging the underlying material.
If it’s a large area, a professional should perform the remediation. They’ll setup containment and HEPA filtration to ensure the spores don’t spread throughout your home during the cleanup process. If the area is relatively small, you can likely tackle it yourself.
The removal process typically requires the use of both a fungicide and physical removal of the mold. The fungicide will deactivate and kill the mold spores, but staining and discoloration will likely remain. This is often addressed with HEPA vacuuming, scrubbing, steam cleaning, sandblasting, etc. In large commercial settings, dry ice blasting is occasionally used to remove mold from concrete.
A note about expectations
While cleaning the active growth from the concrete is relatively straightforward, some staining will often remain. This is especially true when dealing with black mold growth. The pigment left behind by the growth can be difficult to remove. It’s important to remember, this staining is not a health hazard, nor can it regrow. It’s simply a byproduct of the original growth. If the discoloration remains after the initial cleaning, paint the concrete with a stain blocking concrete primer.
Preventing mold regrowth
If the problem is related to ground water intrusion:
If at all possible, resolve the moisture issue before it hits your concrete wall. This may be as simple as redirecting downspouts away from the affected area or sloping the ground away from the wall. It’s cheap and certainly worth a try.
If those techniques don’t solve the problem, you’re left with two choices – a french drain or interior footing drain. French drains are a great choice during the initial construction of the home or shortly thereafter. There’s little additional excavation because the foundation wall is already exposed.
Once a house is up and running, it’s a very different story. The excavation must deal with plants, trees, driveways, decks – basically anything within a few feet of the perimeter of your home. If it’s a single, unobstructed wall, it can be accomplished. But in many cases it’s cost prohibitive.
For existing homes, an interior footing drain is best. It’s the same concept of a french drain, but it’s installed inside the home, at the base of the concrete wall. In most cases, any framing directly against the wall must be removed first. If moisture is entering higher up the wall, dimple board will be necessary. This is a plastic membrane that directs the water downward toward the footing drain.
What about sealants?
In general, sealants and coatings are ineffective against basement water intrusion issues. If you have substantial water pressure from the outside, it will find a way in your home. They can be successful in scenarios where you have a single leak from an obvious crack.
If the mold problem is due to humidity:
Lower the relative humidity. Concrete is a poor insulator and therefore can become quite cool during the winter months. In a poorly ventilated basement, this can cause airborne moisture to condense on the surface and cause mold growth. There are three ways to reduce the RH in the area.
- Increase the temperature. The warmer the air in the room, the lower the RH. Now remember, if the temperature of the room increases, but the temperature of the concrete wall remains the same, the problem will remain. This is due to the dramatic increase in RH that occurs in the air immediately adjacent to the cold wall. However, in a fairly mild climate, the increased temperature of the air will sufficiently increase the temperature of the wall to prevent condensation from occurring.
- Improve the ventilation. Assuming the condensation occurs during the winter months, bringing in fresh air from the outside will reduce the RH inside the building. This is because warm air can hold much more moisture than warm air. So if we bring wet cold air into the home through ventilation, and heat it up, the net level of humidity will drop. It seems counter intuitive, but this can dramatically reduce the moisture load within the basement.
- Dehumidify. This isn’t our first choice, but occasionally it is the only option. There are a couple of reasons I don’t often recommend this technique. First, dehumidification requires a significant amount of electricity. Second, while it reduces the moisture in the air, it does nothing to improve the quality of the air. Musty odors, chemical fumes, VOCs, etc. will all remain in place if dehumidification is used. Alternatively, if the ventilation is improved, both the RH and the air quality will improve.
Case Study > Mold on Concrete in a Basement
This project is a perfect example of the compounding factors of poor ventilation and uninsulated concrete walls. The RH in the basement was 80%. This is very high. Mold will begin growing on random contents at this level.
80% RH is problematic even in well insulated walls. In a room with concrete walls, it’s guaranteed to cause mold growth.
Increase ventilation by installing a constant flow exhaust fan in the bathroom. These units replace a regular bath fan, but operate 24/7 at a reduced speed. They provide continuous ventilation throughout the day.
- RH: 80% Temp: 56 deg F. CO2 = 545 ppm CO = 0 ppm
- 15-18 inches of standing water was present throughout basement.
- Virtually every surface (wood paneled walls, ceiling, subfloor joists, concrete etc…) was covered with mold growth.
- All appliances were submerged and required removal.
- Sump Pump was present but was not running due to the power being cut off to the home.
- No ventilation to basement has trapped humidity and has allowed mold growth to occur on all surfaces.
- Mold growth was observed behind all ceiling panels on car-decking subfloor and ceiling joists.
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My basement leaked during heavy rain and a towel was left on the floor. Upon removing the towel, the floor was slimy and black. How do I clean it up?
Assuming the floor is concrete, spray the area with a Borax/water solution. Clean up the solution and any debris with paper towels.
Occasionally the mold growth will leave behind a stain, even after a vigorous cleaning. The staining is an aesthetic rather than a health issue. If this occurs, steam/pressure cleaning may be necessary.
My concrete mold is located on the front corner of step outside. It looks like harden yellow substance?
If it’s hard to the touch, it is likely efflorescence, not mold growth. Occasionally, efflorescence can appear in different colors (see photo above with orange deposits). This is due to an interaction of the minerals within the concrete.
Our basement cellar has a substance growing on it. Is this mold?
Yes, this looks like mold growth. It is quite common for cellars to suffer from mold growth. This is due to the cool temperatures and high humidity. Regularly cleaning the concrete can help (which admittedly is not a fun activity). Also, painting the concrete with semi-gloss, mold resistant paint will make it easier to clean. Depending on your climate, a long term solution might involve a dehumidifier or increased ventilation.
Picture of hard deposits (or growth?) on inside of concrete blocks. It's only on the lower 1-2 courses of block. Looks a bit like potential masonry cement spill but this doesn't feel plausible. Drywall just removed had black mold on the inside of the house surface. Insulation in block and drywall had little or no mold evidence. Should I have a sample taken and get the deposit tested?
This is a tricky one. If it’s hard to the touch, it’s not mold. It’s either mortar or efflorescence. The latter will dissolve in water. Typically it’s powdery, but I’ve seen it form hard, crystal like deposits in some cases. The fact you found mold on the backside of the sheet rock points to an ongoing moisture intrusion. This would explain the efflorescence, which occur when moisture moves through masonry. I recommend testing the deposits by submersing them in water. Do not reinstall the sheet rock until you’ve dealt with the underlying moisture issue.
I found a hard black coating under painted concrete in my basement. If after cleaning can I paint over it. It did not all scrub up. Does mold get imbedded in the cement.?
Based on your description, this does not sound like mold. On concrete, mold will appear in either a fuzzy format or a stain like appearance. It would not feel like a hard coating. In either scenario, it is fine to paint the concrete after you’ve thoroughly scrubbed and cleaned the area. Anything remaining is simply the pigment from the prior mold growth, which cannot regrow.
There has been a wet smell In our newly purchased home since we got here. The previous owners did not heat the home in the winter months and there is very poor ventilation in the basement. We removed some Gyproc today..does it look like we will have to remove it all?
Yes, the sheet rock should be removed. This is necessary not only to clean up the existing mold, but to implement the prevention measures.
First, you need to identify the source of the moisture. Three possibilities exist. 1.) Condensation from the warm interior air hitting the cold concrete wall. This typically does not happen behind a well sealed wall (i.e. sheet rock), but it’s possible.
2.) Vapor emissions through the concrete. This occurs when moisture moves through the concrete in vapor form, creating high humidity and saturation on the surface. In an uncovered wall, the water would simply evaporate into the air without causing an issue. The installation of the sheet rock traps the moisture, leading to mold growth. There are two solutions – either keep the concrete exposed or install dimple board with a interior footing drain.
3.) Liquid water intrusion. It’s possible you’re having liquid water enter through the cold joint at the bottom of the wall. If this is the case, you’ll see more damage at the bottom plate/framing than higher up the studs. In most cases, the solution is an interior footing drain.
We have a problem with a room in our basement. It is uninsulated and has a moldy smell. The odor is worse in the winter months. My son likes to sleep in the room - what should I do?
You’ll want to focus on two items – increasing the temperature and the air flow. Addressing these issues will lower the humidity and the mold growth/musty odor.
Purchase a relative humidity gauge and install it in the basement. Your goal is to keep the RH below 50%. Increase the heat as necessary to lower the RH until you’ve reached 50%.
The easiest way to increase the ventilation is to run a bathroom exhaust fan. If there’s an existing fan in the basement, run it at least 12 hours per day in the winter months.
It’s also important to verify whether the basement is suffering from vapor emissions through the concrete slab. This occurs when water vapor moves up through the slab and into the room above. If the basement is carpeted, the moisture can become trapped and lead to mold growth. See our page on basement mold for more info.
My basement wall has developed this in recent months. This is a shared wall of a twin home, but this part of the wall extends past the neighbor's house and I believe there is water sitting on the side of the house. Is this mold?
This is likely a combination of mold and efflorescence. Efflorescence occurs when moisture migrates through the concrete, picking up minerals along the way. Once the moisture reaches the surface, the water evaporates, leaving behind the mineral deposits. Because this is a product of water movement, often a small amount of mold growth is present as well. If the concrete wall is unfinished/uncovered, it will typically dry out on its own. However, you’ll need to fix the issue if you plan on finishing out this wall with framing, insulation, etc. The wall will inhibit dry out and lead to mold and rot damage to the framing.
Preventing moisture migration through a concrete foundation wall is accomplished by a combination of the following. 1.) redirecting downspouts away from the home. 2.) Installing a waterproof membrane on the interior side of the foundation wall. 3.) Installing an interior footing drain and sump pump at the base of the foundation wall.
We recently had a concrete shower installed 30-45 days, and these black spots have appeared. The shower does not properly drain, so water sits on the shower floor typically until we shower again. Is it mold?
Yes, this looks like light mold growth. While this may be aesthetically unpleasant, it’s not likely affecting the indoor air quality. Concrete can be surprisingly porous, depending on how it cured. This porosity allows water to penetrate and can lead to mold growth. Prevention is best achieved by treating the concrete with a penetrating sealer. Note – this should be done relatively soon, as the concrete loses its ability to absorb the sealant over time.