Efflorescence on Basement Walls: What the White Powder Means

What is the white powder on basement walls?

It’s efflorescence: salt that water carried through the concrete and left on the surface when it evaporated. InterNACHI defines it as dissolved salts deposited on the surface of a porous material (opens in a new tab), visible after the water that carried them dries.

The water usually comes from the soil outside. InterNACHI says the moisture behind efflorescence often comes from groundwater, but rainwater can also be the source (opens in a new tab). Concrete is porous, so water wicks through it and brings salts from the concrete and the soil along.

That’s why the white powder on basement walls shows up on your side of a below-grade wall. The water comes in from the soil face and dries at the basement face, where the air is drier.

Is efflorescence a problem?

Not by itself. InterNACHI calls it a cosmetic issue, but it indicates a potential moisture problem (opens in a new tab). The salt doesn’t hurt anything; the water that keeps delivering it can.

The National Park Service’s masonry glossary says efflorescence may be more unsightly than harmful (opens in a new tab), but that it often serves as a warning that water has found a point of entry into the structure. Once that happens, it says, more serious damage can usually be predicted.

The damage worth knowing about is salt that crystallizes inside the concrete instead of on it. The same glossary calls this subflorescence and says that, during freeze-thaw cycles, the pressure it builds may cause parts of the outer surface of the masonry to spall off (opens in a new tab). Flaking or crumbling under the white powder can be a sign of salt building up inside the wall.

Is it efflorescence or mold?

Rub it and wet it. Efflorescence crushes to powder and dissolves in water; mold does neither. InterNACHI gives these tests for telling the two apart:

InterNACHI, Efflorescence for Inspectors

Both point to the same cause. InterNACHI says efflorescence generally occurs where there is excess moisture (opens in a new tab), a condition that also encourages mold, so look for mold on wood, drywall or stored boxes near a wall with efflorescence.

If you do find mold, the EPA’s rule is that the key to mold control is moisture control (opens in a new tab). Its guide says most people can clean up an area smaller than about 10 square feet themselves, and points to its remediation guidance for anything larger.

What does the pattern tell you?

Salt forms where water dries, so its shape traces the water’s path. Read it before you scrub it off, and take a dated photo.

• A line along a crack. Water is coming through the crack. That’s a leaking crack, and the leaking crack guide covers sealing it.
• A band low on the wall or along the floor. Water is wicking up from the soil. InterNACHI notes that footings are typically poured directly onto soil without any capillary break (opens in a new tab), so water can wick inward and upward.
• A patch below a window or one section of wall. Look outside above it for an overflowing gutter, a downspout or a low spot that holds water.
• White deposits on mortar joints in a block wall. Water is moving through the block. Check the outside for the same sources.

How to remove efflorescence

Start dry and gentle, and use acid only as a last resort. Each step adds risk, so stop at the first one that works.

1. Brush it off dry

   Use a stiff brush. InterNACHI lists simply scrubbing efflorescence off with a strong brush (opens in a new tab) as a removal method, and a dry brush puts no new water into the wall.

2. Vacuum the dust

   Vacuum the wall face and the floor below so the loose salt doesn’t get wet and soak back in.

3. Use water sparingly, then dry the wall fast

   Efflorescence is water soluble, so a damp sponge or water rinse can lift what’s left. InterNACHI warns that water can let the crystals re-absorb into the host material (opens in a new tab) and reappear, so dry the wall quickly with fans or a dehumidifier.

4. Acid only for stubborn deposits

   Use a masonry cleaner made for efflorescence and follow its label for dilution, ventilation and protective gear. The safety steps from the sources are below.

If you use an acid cleaner, InterNACHI says to wet the wall with water first, neutralize with baking soda afterward (opens in a new tab), and keep muriatic acid off your skin and out of your eyes because it’s toxic. The National Park Service recommends a test patch and checking the rinsed surface with pH paper, rinsing again until all residues have been removed (opens in a new tab).

Acid can make the problem worse. The same Park Service cleaning guide says hydrochloric (muriatic) acid tends to form water-soluble salts in the masonry that even thorough surface rinsing is unable to remove (opens in a new tab), and those salts can come back out as more efflorescence. Its Preservation Brief adds that the acid can dissolve lime-based mortar (opens in a new tab), which matters on a block wall.

Don’t acid wash in cold weather. The Park Service says chemical cleaning should never be done below 40°F, and generally not below 50°F (opens in a new tab), because the water in it can freeze.

How do you stop efflorescence from coming back?

Stop the water. The National Park Service says that with efflorescence from water getting into masonry, the first task is to identify the point of entry and stop the water penetration (opens in a new tab). Cleaning without that is a chore you’ll repeat every wet season.

Start outside, on the wall where the salt shows:

• Grade. The soil should slope away from the house. The NRMCA recommends a finished grade that slopes 1/2 to 1 inch per foot for at least 8 to 10 feet (opens in a new tab) from the foundation, kept that way as the backfill settles.
• Downspouts. Carry roof water away from the wall. A Canadian building research digest recommends extensions to the downspouts and splash blocks (opens in a new tab).
• Gutters. Clear them and watch them in a heavy rain for overflow over the wall with the salt.
• Window wells and paving. A window well that fills with water or a patio sloping toward the house sends water straight to the wall.

Then deal with any crack that’s leaking. A salt line along a crack in a poured wall means water is coming through it, and polyurethane foam injection seals it through the wall’s full thickness.

The leaking crack guide explains which sealer works on a wet crack, and the injection guide walks through the job. The kit’s instructions say to wire brush off efflorescence before setting the ports.

Should you seal a basement wall that has efflorescence?

Not the inside face of a below-grade wall that gets wet. Fix the water first. A sealer on your side doesn’t stop water coming from the soil side, and it can hold moisture and salt in the wall.

The National Park Service’s guidance on water repellents says a water-repellent coating should never be applied to a damp wall (opens in a new tab), and that it won’t solve rising damp and may make it worse. It also warns that salts trapped under the surface can cause spalling, particularly if a water-repellent coating has been applied (opens in a new tab). The Flex Seal review makes the same point about coating the inside of a wet wall.

PS110 fits the above-grade side of the problem. Its data sheet lists combating efflorescence among its uses and recommends it for home exteriors, block walls, brick walls, chimneys and retaining walls, on horizontal or vertical, interior or exterior surfaces. The same sheet says it’s not intended for below grade waterproofing, not intended to seal cracks, and shouldn’t go on wet or damp surfaces.

Wait until the white deposits have stopped coming back, since salt left in the wall is what makes a repellent risky. Then use it where it fits: the exposed part of the foundation above the soil, block or brick walls, retaining walls and chimneys, on a dry day after you’ve cleaned off the salt. The data sheet says to neutralize the surface completely if you used acid to clean it, apply between 40°F and 95°F, and keep it off surfaces where rain is expected within 12 hours.

PS110 is a penetrating sealer, so it doesn’t form a film and its sheet says it preserves the concrete’s breathing. The penetrating vs topical sealer guide explains why that matters, and the sealing hub covers every other surface. White haze under a film sealer is a different problem, covered in why concrete sealer turns white.

When is efflorescence a sign of a bigger problem?

When it comes with a crack that’s a structural warning sign. The salt only tells you water is moving; the crack tells you whether the wall is. Have a structural engineer look before you seal or coat anything if you see:

• A horizontal crack in a basement wall. The horizontal crack guide shows what it means.
• A wall that bows, leans or bulges inward. See the bowing wall guide.
• A wall crack wider than 1/4 inch.
• A crack with offset edges, where one side sits farther out than the other.
• A stair-step crack in a block wall. The stair-step crack guide covers it.

Flaking, crumbling or spalling concrete under the salt is also worth a closer look, since the Park Service links it to salt and moisture inside the wall. For every other crack type, start at the foundation cracks hub.