White Substance On Ducts and Metal???

Hey Guys-

This was on a home built in the 70s with a half basement half crawlspace. The ductwork running through both were randomly covered in a white powder substance. It had a similar texture to joint compound or sheetrock mud. At first I thought maybe it got on the ducts when they were being made or stored, but I then noticed in the crawlspace that it was also on the metal plumber’s tape, joist supports, and a little on the masonry blocks. The crawlspace had no ventilation and was completely dry. Any ideas what it might be from???

Search it.

This has been covered before…

Too much moisture is a place to start.

Found the post:

I should have looked harder before posting…thanks.

Uncovered dirt crawl space is my first clue.

The mortar joints appear to be efflorescence. I have seen asbestos duct wrap and removed this product in the past. The material is removed by wetting the material and scraping it of with a flat scraper. When done halfassed, residue remains. A closer look would be needed to determine if this is that product. If not, it’s a moisture issue.

Bob, could you explain further. First clue for what…moisture problems?

Linus, did you look at the forum link I posted above:

In that scenario it seems it was simply a moisture problem during storage of the sheets and only cosmetic problem. But it seems you’re indicating it might be an asbestos residue issue. As I looked again at the previous post, they discussed not being able to scratch it off. Whereas when I tried it, it scratched off as a powdery substance. Which makes me think maybe it’s more like what you’re talking about. Did they still use asbestos wrapping in 1977? I know you only have pictures to work with, but do you have thoughts on which it might be?

Kenny, is that un-insulated duct work in the crawlspace?

Yes. Is there more to that question?

Kenny, there is an excellent training video on NACHITV, Advanced Inspection of Crawlspaces.


Insulation contractors installed their inventory of asbestos containing products until they ran out of it. I have been on hundreds of asbestos abatement projects where both products(ACM and Non ACM) were installed by the same contractor. They didn’t throw out good insulation just because the government banned the use of it.

No moisture barrier (condensation)

Here are some more pics:



I agree to the moisture issue rather than asbestos…I havn’t seen anything built in the 70’s that still had ductwork coated with that stuff.

I worked in the asbestos abatement industry for 15 years beginning in 1987 and saw lots of it on ductwork.