Three field tests you can do in five minutes
| Test | Efflorescence | Mold |
|---|---|---|
| Spray with water | Dissolves and disappears, then returns in the same place later | Smears, darkens, does not dissolve |
| Texture between fingers | Gritty, crystalline, like fine salt or sugar | Soft, fibrous, cottony or slimy |
| Location | Only on masonry: block, brick, concrete, mortar | On masonry and also on wood joists, subfloor and insulation |
| Smell | None | Often musty, earthy, or ammonia-like |
| Pattern | Follows mortar joints and evaporation lines | Spreads outward in irregular patches from a moisture source |
| Color | White to off-white only | White, gray, green, brown or black |
The most useful single test
Spray it with plain water from a spray bottle. Efflorescence dissolves and the wall looks clean for a moment. Mold gets darker and smears. It takes ten seconds and it is right far more often than a visual guess.
What efflorescence actually tells you
What we found
Half the cities we serve have block foundation walls, which is the surface that produces efflorescence. The other half mostly do not.
Of the 20 Metro Atlanta cities in our dataset, 10 have concrete block or parged block foundation walls as a documented dominant condition: Marietta, Smyrna, Sandy Springs, Dunwoody, Brookhaven, Tucker, Norcross, Lilburn, Acworth and East Point. Block is porous and holds soluble mineral salts, which is exactly what produces efflorescence when water moves through and evaporates. On the pre-1940 brick pier-and-curtain-wall stock intown, in Decatur and in Historic Norcross, white deposits on masonry are less common and white growth is more likely to be biological. Where you live changes which of the two you are probably looking at.
Method: We tallied the documented foundation type for each of the 20 Metro Atlanta cities we serve, August 2026. The per-city detail is on each city page.
Efflorescence is not a defect in the masonry and it is not harmful in itself. It is a receipt. Water entered the block or concrete, dissolved soluble mineral salts inside it, migrated to the surface and evaporated, leaving the salts behind. That means water is moving through your foundation wall in measurable quantity, repeatedly. On a Metro Atlanta crawl space wall, that usually means one of three things: grade sloping toward the foundation, roof water discharging at the wall, or a water table high enough to load the base of the block.
The heaviest efflorescence we see locally is on parged block stem walls in the 1968 to 1985 suburban stock, particularly through Dunwoody, Tucker and Lilburn, where the exterior parging has cracked along the mortar joints after forty years and lets water into the block cores. It shows as a vertical band on the interior of the wall, which is a useful diagnostic pattern: it tells you where the water is getting in.
What white mold on concrete tells you
Mould growing on masonry means the surface has held enough moisture, long enough, to support biological growth, and mould on concrete rarely stays on concrete. Masonry is not a food source in the way wood is, so growth on a foundation wall usually indicates humidity high enough that the wood nearby is also at risk. The EPA's threshold is below 60 percent relative humidity, ideally 30 to 50 percent. When we find white growth on a Metro Atlanta crawl space wall, the joists within a few feet of it almost always have growth too, and the joists are the part that matters structurally.
The commonly used phrase white mold is not a species. Many molds appear white during early growth and darken as they mature and sporulate, so white growth frequently means recent or actively expanding growth rather than a distinct or less serious organism.
What to do about each
If it is efflorescence
- Do not just clean it off. It will return, because the water is still moving.
- Walk the exterior of that wall. Check the grade slope, look for a downspout discharging within a few feet of the foundation, and look for a low spot that ponds.
- Correct the cheap causes first: downspout extensions at $120 to $400 each, and regrading at the foundation at $8 to $22 per linear foot.
- If the wall keeps producing efflorescence after the exterior is corrected, the source is groundwater and you need interior perimeter drainage with a sealed sump.
- Only after the water is managed is it worth removing the deposit itself, which is a wire brush and a masonry cleaner.
If it is mold
- Leave it in place until it has been inspected. The growth pattern identifies where the water is coming from, and cleaning destroys that information.
- Check the wood. Take a flashlight and look at the joists and subfloor nearest the growth, because that is where it matters.
- Put a hygrometer in the crawl space for two weeks. Above 60 percent relative humidity, growth on wood is sustainable.
- Plan on remediation plus moisture control together. Treatment without a humidity fix returns within one to two seasons.
- Do not use bleach on wood. It is largely water and its active component does not penetrate porous wood, so it lightens the surface without removing the growth beneath.
What it costs
Efflorescence itself costs nothing to address; the water causing it is what you pay for. Exterior downspout and grading correction runs $120 to $400 per downspout and $8 to $22 per linear foot of regrading. If the source is groundwater, interior perimeter drainage runs $45 to $95 per linear foot with a sealed sump at $1,200 to $2,600.
Mold remediation runs $2.00 to $4.00 per square foot for HEPA vacuuming with antimicrobial treatment, and $3.50 to $6.50 for abrasive blasting where growth is heavy. The moisture control that prevents recurrence is quoted separately and is usually the larger number, at $2,800 to $5,000 for sealed vents and a dehumidifier on a crawl space that already has a sound vapor barrier.
Frequently Asked Questions
Is fluffy white mold on concrete dangerous?
The more useful question is what it indicates. White growth on a foundation wall means humidity high enough to support biological growth, which means the wood framing nearby is also at risk, and the wood is the part with structural consequences. For occupants with asthma or allergies, elevated microbial growth under the living space is worth addressing on its own terms. First, confirm it is actually mold rather than efflorescence.
Can I just paint over white growth on my crawl space wall?
No. Painting over efflorescence traps the salt migration behind the coating and the paint eventually blisters off. Painting over mold covers a living colony that continues to grow on whatever moisture is still present. Both cases hide the diagnostic information without addressing the water, and both cost more to correct later than doing it properly now.
Why does efflorescence keep coming back after I clean it?
Because cleaning removes the deposit, not the mechanism. Water is still entering the masonry, dissolving salts and evaporating at the surface. As long as that continues, the deposit reforms. Efflorescence returning in the same place after cleaning is actually useful confirmation that the water problem is ongoing rather than historical.
Does white mold mean the crawl space needs to be encapsulated?
Not automatically. It means the humidity is high enough to support growth, and encapsulation is one way to fix that, but the right answer depends on the source. If water is entering the wall, drainage comes first. If the ground is bare soil evaporating moisture, a vapor barrier may resolve it far more cheaply. Take readings before buying a system.
Should I test the mold to identify the species?
For a crawl space, species testing rarely changes the plan. The treatment is the same regardless of organism: physical removal, drying the wood below 16 percent moisture content, and fixing the humidity. Testing has real value in a health-related dispute, an insurance claim or a legal matter. For a homeowner deciding what to do next, the money is better spent on a hygrometer and an inspection.
Sources
Primary sources only. This site never cites a home-services marketplace, cost aggregator or competing contractor, because those businesses compete with this one and their figures rarely trace to anything.
- U.S. EPA: A Brief Guide to Mold, Moisture and Your Home. Keep indoor relative humidity below 60 percent, ideally 30 to 50 percent.
- U.S. EPA Mold Course, Chapter 2: Why and Where Mold Grows. Relative humidity above 60 percent is likely to cause condensation, which leads to mould growth.
- CDC: Basic Facts about Mold and Dampness. Health context for damp indoor spaces and mould exposure.
- USDA Natural Resources Conservation Service: Web Soil Survey. Parcel-level soil series data, including the Cecil, Pacolet, Madison and Appling series across the Georgia Piedmont.
- Georgia Structural Pest Control Commission Rules, Chapter 620-6: Wood Destroying Organisms. Conditions conducive to infestation include cellulosic debris under a building, wood-to-soil contact, and under-floor ventilation not meeting the residential code.
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