White powder on foundation walls

Editorial team · Updated September 19, 2026 · How we research

That white powder is mineral salt left behind by water evaporating out of the masonry. It is harmless to touch — and it is proof water is moving through the wall, which is not harmless at all.

Efflorescence is one of the few crawl space findings that is genuinely harmless in itself and genuinely useful as information. Most people react to it backwards — scrubbing it off and moving on, when the interesting part is what it tells you.

What it is

Water moves through concrete block, brick or mortar, dissolving mineral salts as it goes. When it reaches the surface and evaporates, the water leaves and the salt stays.

The white crystalline deposit is that salt. Not alive, not a health concern, and it brushes off with a stiff brush.

But it only forms where water has moved through the masonry and evaporated out of it. That makes every patch a record of a water path, and the pattern is readable.

Telling it apart in ten seconds

If it is on wood rather than masonry, it is not efflorescence and the white growth guide is the one you want.

Reading the pattern

This is where it earns its keep.

At the base of the wall, low band. Splash-back from rain hitting the ground, or a high water table wicking up. Check grading and downspouts first.

Partway up the wall, horizontal band. Water is entering at that height, or standing against the outside of the wall to that depth. This points to grading, a blocked drain, or saturated backfill — and a band at a consistent height often marks the level water reaches outside.

Around a specific block or joint. A localised defect — a cracked block, a failed mortar joint, a penetration. Usually the cheapest kind to fix.

On the underside of the floor structure or on piers. Water is moving up through supports. Worth checking pier condition.

Heavy and widespread. Sustained water movement through the whole foundation. This is a drainage project rather than a repair.

Alongside a horizontal crack. Read them together — a horizontal crack indicates lateral pressure, and efflorescence confirms water is part of what is producing it. That combination warrants an engineer rather than a brush.

Why sealing it is usually the wrong first move

Waterproofing paints and masonry sealers applied on the inside can stop the deposit appearing. They generally do not stop the water.

Two consequences follow. You lose your indicator — the thing that was telling you where water moves. And moisture continues accumulating within the masonry, which in freeze-thaw conditions can spall the face of the block as trapped water expands.

Sealers have a place, after drainage is corrected and as part of a system. As the first response to a white patch, they are cosmetics applied to a diagnostic.

What to do instead

1. Photograph it and note the height above the floor and the location on the wall
2. Brush it off — you want to know whether and where it returns
3. Check the outside at that point: grading, downspouts, hardscape, anything holding water against the wall
4. Wait for rain, then look again. Where it returns first is where water enters
5. Fix the water path — often grading and downspouts, occasionally a drain
6. Re-check in three months. No return means you found it

Step 2 and step 4 together are the whole diagnostic, and they cost nothing.

When it is worth more than a brush

Escalate if the efflorescence comes with a horizontal crack or a bowing wall, if it covers most of the foundation, if it returns within days of removal, or if block faces are flaking and crumbling where it appears.

That last one — spalling — means the masonry itself is deteriorating rather than just carrying salt to the surface, and that is a repair rather than a drainage adjustment.

Costs

As with most things under a Tennessee house, the top of that list solves more cases than the bottom of it — provided somebody actually looks before quoting.