Does anyone know what that white powder at bottom of walls in basement is for; insects comes to mind but…
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Looks to coincide with the hollow cores of the block wall.
Is that inside the firebox of a chimney/fireplace?
Is there a paint/coating on the wall?
Concrete foundation wall, no chimney/fireplace present. It looks like someone applied white powder on the tar paper at bottom of foundation wall. My thought is that it was applied due to the presence of insects but I’m not sure. That second picture showed up sideways…
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I agree. …
2nd picture upright
My initial thought was some sort of microbial growth or prevention treatment based on the way to water stains on the stud . I’m wondering why the tar paper on the concrete…
Were you able to get a sample of the powder or get your hands on it?
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Microbial growth makes sense also. Tar paper (roofing felt) was common in the year that house was built (1991) around here anyway and no I didn’t taste it…
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Possibly lime.
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That is my thought, too, Marcel.
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Did Tony Montana ever live there ?
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I agree with the insecticide thoughts as well.
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Won’t know for certain until a lick test results come in. All just a guess until then.
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That is the word I was looking for, thanks!
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Diatomaceous earth, DE), is a fine powder manufactured from fossilized shells of diatoms,
Diatoms preform, photosynthesize, the conversion of dissolved carbon dioxide in the water into oxygen. Diatoms are a primary food source for higher organisms in the food chain like invertebrates and small fish.
Glad I could help, Marcel. 
Think you should focus on the water intrusion rather than the cocaine type substance…
Staining indicates a problem (your job), powder indicates it is actually a problem.
Powder identification, not your job…
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I already commented on the water intrusion and recommended asking the homeowner the reason for the white powder application in the report. The powder identification was in the event someone was familiar with its usage and also for personal knowledge. Thanks everyone for your comments!
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I too was boggled when I came across this powder in a crawlspace. There was certainly efflorescence on the foundation walls, but this was just odd as it appeared to have been methodically placed, yet random. Diatomaceous earth makes sense. I just can’t believe how it managed to maintain a thick layer on a vertical wall. Diatomaceous earth is also used in pool filtration systems, so yeah, it’s safe.
Lime of Borax would be my first thoughts. Usually Lime is all across the floor due to a sewage leak. Borax would be an insect repellant.
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