You shine a flashlight into the crawl space or along a basement wall and see it: a white, chalky film, a fuzzy crust along a crack, or a crystal line near the floor. It looks alarming. It may be efflorescence, a mineral deposit left behind by water.
The deposit itself is the minor part. What matters is what it tells you about water moving through the wall. This post covers the chemistry, what the pattern can and can’t suggest, how to tell it from mold and other white stains, and what to record before anyone looks at the house.
This is general education for La Jolla homeowners citywide. It doesn’t cover repair methods or drainage design, and it isn’t an assessment of any property.
What efflorescence is
The Brick Industry Association’s Technical Note 23A puts it in one line: “efflorescence occurs when water containing dissolved salts is brought to the surface of masonry, the water evaporates and the salts are left on the surface.”
The Concrete Masonry & Hardscapes Association (formerly NCMA) explains the same process for concrete block and concrete. Its TEK 08-03A lists three conditions that all have to be present: soluble salts, water to carry them, and a path and driving force to move that water to the surface. Then it adds the key point: “If any one of these conditions is eliminated, efflorescence will not occur.”
That’s why efflorescence is useful as a clue. Salts can sit in a wall for decades without showing. When they show up, water has been moving.
Where the salts come from
The salts can come from the wall materials or from outside them.
- Cement products. CMHA describes calcium hydroxide in cement-based materials reacting with carbon dioxide to form calcium carbonate, a white deposit. BIA’s table of efflorescence compounds lists calcium carbonate with “mortar or concrete backing” as a likely source.
- The soil. For below-grade walls, BIA writes: “Soluble salts in soil are dissolved by water that percolates through the ground.” It also notes groundwater “may rise through capillary action several feet above the ground.”
- Seawater. BIA’s table lists sodium chloride with seawater as a source. On a coastal property that’s one possibility among several, not a conclusion. BIA notes that laboratory analysis can identify which salts are present.
What its age and location can suggest
BIA and CMHA both treat timing and placement as evidence.
Age of the wall. BIA describes “new building bloom” shortly after construction, which it attributes to “normal evaporation of water after construction is complete.” Later on, the note says: “When efflorescence occurs more than a year after construction is complete, it can generally be attributed to excessive water penetration or poor drainage.” CMHA agrees that late-age efflorescence “indicates a need for corrective measures.”
Something new. BIA: “The appearance of efflorescence on an established building that had previously been free of efflorescence is usually attributable to a new source of water in the masonry.” Among the possible sources it names are “poorly directed landscape sprinklers, leaky pipes, faulty drains.” If the white powder showed up after a landscaping change or a plumbing repair, write that down.
Where on the wall. BIA says location “may offer some information as to where the water is entering.” CMHA notes that streaks or a localized patch “indicates that water has entered or is entering the wall at a higher elevation.” For deposits low on the wall it says: “If the efflorescence is near ground level, ground water may be the cause.”
How much and how persistent. The NIH Office of Research Facilities bulletin on masonry walls warns that “Persistent, repeated or excessive efflorescence may indicate excessive internal moisture, rising damp or other problems which should be investigated.”
Treat these as hints, not diagnoses. A white line tells you water reached that spot and dried there. It doesn’t map the whole path.
What efflorescence doesn’t mean
On structure, the sources are reassuring about the deposit itself. CMHA states: “Although it may be an aesthetic concern, efflorescence will not affect structural performance.” BIA writes that “efflorescence is usually not harmful to brick masonry.”
There are two caveats. First, BIA notes that when salts crystallize inside the masonry rather than on the face, they “may cause cracking and distress to masonry.” Second, the water behind the powder is a separate issue. HUD’s Residential Rehabilitation Inspection Guide lists “dampness, water stains, peeling paint, efflorescence, and rust on exposed metal parts” together as signs of water penetration, and tells inspectors: “Determine the source of any moisture that may be present.” HUD’s list is a reminder that the same water can reach wood framing and metal connectors.
Efflorescence beside a crack doesn’t mean the crack is structural. It means water has used the crack as a path. Whether the crack itself matters is a separate question. Our foundation cracks page and the post on hairline vs structural cracks cover that.
Why covering it up isn’t the answer
Since the deposit is a symptom, hiding it doesn’t remove the cause. CMHA writes that “the problem should be repaired prior to removing the efflorescence,” and warns that “The use of water to remove surface accumulations, including efflorescence, will cause additional water to enter the wall.” BIA cautions that washing “may bring more salts to the surface.”
BIA is also wary of coatings. They can trap salts beneath the surface, causing “cryptoflorescence or subflorescence,” and so “coatings are not recommended as the sole treatment for efflorescence problems.” The sequence the sources describe is: find the water first.
Efflorescence, mold, or something else?
White deposits aren’t always salt. BIA notes: “A number of stains that affect brick masonry are white in color and may be confused with efflorescence.” And mold can look similar. The U.S. EPA’s mold course says: “Alkaline crystals on soil or concrete walls may look like mold, but, unlike mold, they are usually water-soluble.”
North Dakota State University Extension describes a simple field check: “Putting a few drops of water on the substance is one way to determine if it is salt or mold.” It adds that carbonate deposits are “relatively insoluble in water,” and suggests vinegar as a second check: “If the substance does not dissolve in either water or vinegar, then it may be mold.” A few drops on a small spot is enough. Soaking a wall adds the water you’re trying to understand.
If you’re unsure what you’re looking at, treat it as a moisture question and have it looked at. Our post on wet crawl space warning signs covers the other moisture cues under a house.
A short note on the code
The San Diego Residential Code requires dampproofing on foundation walls that retain earth and enclose interior spaces below grade, and waterproofing “In areas where a high water table or other severe soil-water conditions are known to exist.” It also addresses foundation drains for below-grade space. Efflorescence on a wall doesn’t prove any of those measures are missing or failed. It tells you water got through somewhere, and whoever evaluates the wall can take it from there. For how moisture paths around La Jolla foundations get sorted out, see coastal moisture and foundation drainage.
What to write down
- Photos with dates, close-up and wide, so the location on the wall is clear. Our crack monitoring guide has tips.
- Where it is: low near the soil, along a crack or joint, under a window or pipe, or spread evenly.
- When it appeared and whether it comes back after you brush it off.
- Recent changes: new sprinklers or planting beds, a plumbing leak, a downspout moved, heavy rain.
- Other moisture signs nearby: damp soil, stains, peeling paint, rusted metal, musty smell.
- Water or vinegar check result, if you tried a few drops.
Cost factors, not prices
We don’t publish prices. Things that shape the effort and cost of looking into efflorescence include: how easy the crawl space or wall is to reach; how much of the wall is affected; whether the moisture source is obvious or takes investigation; whether lab analysis of the salts is useful; whether cracks or framing near the deposits need evaluation; and whether drainage or plumbing specialists get involved.
Where we fit
We’re a foundation and structural repair company serving La Jolla citywide. On a visit we can look at where the deposits are, what’s around them, and whether cracks or framing nearby need attention. We aren’t engineers, and we don’t treat efflorescence as a diagnosis on its own. When the moisture picture points to a structural or soils question, we coordinate with licensed engineers as needed. Our foundation drainage, crawl space foundation repair, and foundation inspection pages explain those services. This post is general education, not an assessment of any specific property.
Next step
If white powder keeps coming back on your foundation walls, request a Free Quote for an on-site evaluation anywhere in La Jolla, or call (650) 739-6537. Bring your dated photos and notes.
Sources (accessed October 24, 2026)
- Brick Industry Association, Technical Note 23A: Efflorescence – Causes, Mechanisms and Remedies (June 2019)
- Concrete Masonry & Hardscapes Association, TEK 08-03A: Control and Removal of Efflorescence
- National Institutes of Health, Office of Research Facilities, Technical News Bulletin Issue 71, “Efflorescence in Masonry Walls” (December 2017)
- U.S. Department of Housing and Urban Development, Residential Rehabilitation Inspection Guide (February 2000), Section 3.1
- U.S. Environmental Protection Agency, Mold Course, Chapter 3
- North Dakota State University Extension, “Is it Mold or isn’t it?”
- San Diego Residential Code 2025, Sections R405.1, R406.1, R406.2