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.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
Is the black residue on PVC window frames toxic mold?
- 1Scraping off as much as possible manuallyDid this help you?
- 2Wipe it off to see if it comes backDid this help you?
- 3Smell the substance to check for chemical leaksDid this help you?
- 4Applying Goo Gone or Goof OffDid this help you?
- 5Using foam shaving cream and scraping with a razor bladeDid this help you?
- 6Applying vinegar to dissolve hard water mineral depositsDid this help you?
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'm 18 and renting my first place! i just moved in and everything is clean, but ALL my windows look like this at the bottom. I thought it was mold and tried sprayibg witj mold killer and scrubbing, but no matter how hard i scrubbed it looks the exact same? Could someone please tell me what this is and how i fix it?”source ↗
“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
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