What efflorescence actually is
A federal facilities technical bulletin, published by NIH's Office of Research Facilities, puts it plainly: efflorescence is a white or light-colored substance on the surface of concrete, brick, cast stone, and other masonry, made up of soluble alkaline salt deposits that have been carried to the surface by water migrating through the wall. The salts themselves are naturally present in concrete, mortar, and some masonry units. As water moves through the material and evaporates at the surface, it leaves the dissolved salts behind as a visible crust or powder.
That mechanism is the whole story: efflorescence cannot form without water moving through the masonry. Seeing it is direct physical evidence that moisture reached that wall and passed through it, whether or not you can see or feel the water itself right now.
Efflorescence vs. mold vs. other white deposits
This is the single most common misidentification in a crawl space, understandably - both are pale and unfamiliar to most homeowners the first time they see them under a flashlight.
| Property | Efflorescence | Mold |
|---|---|---|
| Where it grows | Concrete, brick, block, masonry | Wood, insulation, organic material |
| Texture | Crystalline, powdery, crushes between fingers | Fuzzy, cottony, or raised - three-dimensional under a low-angle light |
| Water test | Dissolves when sprayed with water | Does not dissolve |
| What it is | Mineral salt - not alive | A living organism |
| Health relevance | None - not a biological hazard | Can affect sensitive individuals; see EPA guidance |
Two quick field checks settle it in a few seconds: try to crush it between your fingers, and spray a little water on it. Efflorescence powders and dissolves. Mold does neither. For the fuller picture on identifying what is actually growing in a crawl space, including decay and ordinary dust, see mold in a crawl space: how to identify it. If the white material is on the wood or the soil rather than the concrete, white mold in a crawl space covers what it usually is and what it says about the moisture.
When it is harmless and when it is not
All masonry develops some efflorescence after construction, as residual construction moisture works its way out of new concrete and mortar. That first-year appearance is expected, generally fades or washes off on its own, and is not a sign anything is wrong.
What changes the picture is persistence. Efflorescence that keeps returning, that appears heavily rather than as a light dusting, or that shows up on a foundation that has been standing for years rather than months, indicates ongoing water movement through the wall rather than residual construction moisture working itself out - and that is worth investigating rather than just wiping away repeatedly.
Why it shows up on Seattle-area foundation walls
Two regional conditions make persistent efflorescence a common enough finding here to be worth its own page. The first is what lies a few feet down. Much of the area sits on glacial till, the dense, relatively impervious layer known locally as hardpan; Seattle's landslide study says water soaking down through the looser soil above perches on top of it, usually less than 10 feet deep, through the winter, and USGS describes till at the surface over most of southwest King County outside the major valleys and exposed over about 40 percent of its east King County study area. A stem wall set into that ground spends months with water standing against it rather than draining past, and water held against concrete is exactly what drives salts through it. The soil survey's ratings do not show this: across most of Seattle the mapped ground is unrated urban land, and the common Alderwood soils elsewhere are rated moderately well drained. Those ratings describe the soil, not the till beneath it.
The second is the age of the housing stock. Citywide in Seattle, 30.1 percent of housing units were built before 1950, per Census estimates (the climate and housing data has every city), and stem walls of that age were built without the wall detailing and drainage practices covered in current technical guidance. Persistent efflorescence on an older foundation here is a plausible, common finding rather than an anomaly.
What it means if it keeps coming back
The deposit itself does no damage. What it is evidence of can, if left unaddressed:
- The moisture behind it feeds other problems. Water moving through a foundation wall is also raising humidity in the crawl space generally, contributing to the conditions covered in high humidity in a crawl space.
- Over the very long term, moisture movement through concrete is associated with other deterioration of the material itself, separate from the visible salt deposit, which is why federal technical guidance treats persistent efflorescence as a signal to investigate rather than only a cosmetic issue.
- It usually means the same water reaching that wall is also reaching the crawl space floor, so persistent efflorescence is often a visible marker for a broader standing-water or drainage problem rather than an isolated wall issue - see standing water in a crawl space.
What you can check yourself
- Run the two field tests: does it crush to powder, and does it dissolve with water?
- Note whether it is light and confined to one small area, or heavy and spread across the wall.
- Check whether it has appeared before, been cleaned, and returned - and if so, how quickly.
- Look at grading and downspouts near that section of wall from outside, the same check relevant to any wall-moisture issue.
- Light efflorescence can be washed off with plain water; avoid sandblasting or aggressive wire brushing, which can damage the masonry surface.
What this does not include: diagnosing why water is reaching the wall, or performing any drainage or waterproofing work. Anyone you pay for that work has to be registered with the Washington Department of Labor & Industries, bonded and insured; how to verify a Washington contractor shows where to check.
What a professional evaluation adds
A professional crawl space inspection identifies where the water reaching that wall is actually coming from - surface runoff, a high water table, or a specific drainage failure - which the deposit itself cannot tell you. That is what decides whether the fix is exterior grading, a full drainage system, or waterproofing the wall directly.
Fixing it
Where water is moving through the wall itself, crawl space waterproofing combines an interior perimeter drain, crack repair on the foundation wall, and a vapor barrier - modeled at $4,025 to $7,175 around Seattle, before sales tax. See the waterproofing cost guide for the full breakdown.
Where the source is water arriving at the wall from the surrounding soil rather than penetrating it, drainage alone may be sufficient, modeled at $4,025 to $7,175.
A free in-home inspection identifies where the water behind persistent efflorescence is actually coming from before you pay to fix it, and you get a written, no-obligation quote. Read any quote for this work for three things: the water source it says it is treating, whether exterior grading, drainage and wall waterproofing are priced as separate lines, and whether the deposits are washed off first so you can see whether they return once the work is done.
Sources
The definition and treatment guidance for efflorescence is drawn directly from the cited federal technical bulletin, written for exactly this material rather than a marketing summary of it. The glacial till descriptions are Seattle's landslide study and USGS's King County groundwater reports; soil drainage classification is USDA's SSURGO survey and the housing-age share is Census ACS data, both as published in this site's climate and housing dataset.
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National Institutes of Health, Office of Research Facilities, Efflorescence in Masonry Walls (Technical Bulletin, December 2017) Checked
What efflorescence is (soluble alkaline salt deposits carried to the surface of concrete, brick or masonry by water migrating through the wall), that it requires water movement through the material to form at all, that a light degree after new construction is normal and not harmful, and that persistent, repeated or excessive efflorescence may indicate excessive internal moisture or other problems worth investigating. Also the removal guidance (gentle washing; avoid sandblasting or wire brushes) and “rising damp” as the term for moisture absorbed from surrounding earth.
Note: A federal facilities-maintenance technical bulletin aimed at building managers, not a peer-reviewed materials-science paper. It is cited because it states the mechanism and practical thresholds plainly and is a primary federal document, not an industry marketing page for a waterproofing product.
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Seattle Public Utilities (prepared by Shannon & Wilson), Seattle Landslide Study, Volume 1 Checked
That Vashon till, known locally as “hardpan”, is very dense and relatively impervious, so water soaking down through the deposits above it normally perches on top of it; that the contact between recessional outwash and till is normally less than 10 feet deep and the water perched there normally dries up in spring or summer and returns in winter; that the boundary where groundwater perches on the Lawton Clay is commonly termed “The Contact”; and that the study’s database holds 1,326 landslides reported through June 1999, records going back to 1890, about 84 percent with some human influence, landsliding in only about 1 percent of the city, and the highest densities in Southwest Magnolia, Southwest Queen Anne, Madrona, Interlaken and Alki.
Note: The PDF prints no date; 2000 is the year Seattle Public Utilities gives for it. A 2003 update added about 110 landslides; this site quotes the original figures.
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U.S. Geological Survey, Hydrogeology and quality of ground water in southwest King County, Washington (Water-Resources Investigations Report 92-4098) Checked
That across the southwest King County study area, which the report describes as including south Seattle, Renton, Kent, Auburn, Des Moines and Federal Way, till (“hardpan”) lies at the land surface over most of the area except the major valleys and averages about 60 feet thick; that many shallow wells in the till held water in winter but were nearly dry by late summer; and that the till plains have poorly drained stream courses and closed depressions holding lakes, wetlands and peat bogs.
Note: A regional groundwater study. It describes the till plains, not any city’s boundary or any lot.
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U.S. Geological Survey, Geohydrology and ground-water quality of east King County, Washington (Water-Resources Investigations Report 94-4082) Checked
That in east King County the till, “often referred to as hardpan or boulder clay,” is exposed at the surface over about 40 percent of the study area, is typically about 70 feet thick, acts as a confining bed, and can hold perched bodies of water in its upper part; and that the Sammamish Plateau is the western of the area’s drift-covered plateaus.
Note: A regional groundwater study. It describes the plateau and the study area, not the City of Sammamish or any lot in it.
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USDA Natural Resources Conservation Service, Soil Survey Geographic Database (SSURGO), via Soil Data Access (muaggatt.drclassdcd — drainage class, dominant condition) Open dataset Checked
The soil drainage columns for the 19 cities in the Seattle-metro climate and housing dataset: the share of each city’s mapped area whose dominant soil drainage class is somewhat poorly drained, poorly drained or very poorly drained, and the share carrying no drainage class at all. Computed on 2026-11-10 by intersecting each Census place boundary with the SSURGO map units of the King County Area, Snohomish County Area, Pierce County Area, City of Seattle, City of Tacoma and Mount Baker-Snoqualmie National Forest survey areas and area-weighting the result; the survey-area versions were read on 2026-11-11.
Note: SSURGO assigns no drainage class to Urban land map units or to water. In Seattle, Shoreline and Burien more than four fifths of the mapped area is unrated Urban land, so their wetter-class shares are near zero because the ground is unrated, not because it drains well, and they are not compared with the other cities. Drainage class also describes natural soil in place — not the fill, the grading or the downspout run at any individual house.
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United States Census Bureau, American Community Survey 5-Year Estimates — Year Structure Built (B25034) and Median Year Structure Built (B25035) (ACS 2020–2024 5-year estimates, tables ACSDT5Y2024.B25034 and ACSDT5Y2024.B25035) Open dataset Checked
The housing-age columns for the 19 Washington cities in the Seattle-metro climate and housing dataset: total housing units, median year built with its published margin of error, and the shares of units built before 1980, before 1960 and before 1950, each summed from the B25034 decade buckets per Census place GEOID. Pulled through the data.census.gov table endpoint on 2026-11-10; for every place the ten buckets were summed and checked against the table total.
Note: Survey estimates carrying sampling error, not a census of buildings. They cover ALL housing units in a place, apartments included, rather than single-family homes — and the ACS does not ask about foundation type, so nothing here identifies which units have a crawl space.
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Washington State Department of Labor & Industries, Register as a Contractor Checked
That Washington requires every construction contractor to register with L&I and to be bonded and insured; that there are two classifications, general and specialty, with 63 specialties; that a specialty contractor works only in the registered specialty and cannot hire subcontractors; that plumbing, electrical, elevator, mobile home installation, boiler and asbestos work carry further education, examination and licensing requirements; the bond amounts ($30,000 general, $15,000 specialty); the liability insurance L&I asks for ($200,000 public liability and $50,000 property damage, or $250,000 combined single limit); that the bond and insurance must carry the exact business name; and that L&I suspends a registration when a bond or insurance policy lapses or a judgment goes unsatisfied.
Note: L&I’s page for contractors. The statute states the insurance minimums differently (RCW 18.27.050); this site gives both.
Sources last checked between and ; the date beside each one is when it was last opened. Where a figure comes from a document that is revised on a schedule, the edition or version is named above so you can confirm it against the current one yourself.