Is the black residue on PVC window frames toxic mold?
Black residue left on PVC window corners after removing old caulk causes anxiety about whether it is active mold, fire damage, or a health hazard. This uncertainty stops homeowners from properly killing the buildup and applying new insulation and caulk.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Scraping off as much as possible manually
- 2Wipe it off to see if it comes back
- 3Smell the substance to check for chemical leaks
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I've removed the old caulk in my bedroom, as it had blackened and the black stuff seem to have gone onto the surface of the PVC window. Is this mould? It's bone dry so should I be concerned as to whether it is alive and how can I effectively kill it before insulating with expanding foam and sealing with caulk?”source ↗
“My main concern was fire damage. My second concern was mold. My irrational concern was I have a bat living up there lol.”source ↗
“It’s not on any of the other fixtures so was concerned that it was something toxic.”source ↗
“Haven’t a clue but concerned it could be dangerous.”source ↗
Where this came up
People with this problem also raised
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