Mold or mildew

Has anyone ever seen growth on an A/C duct like this before. I have seen some like this, but not to this extent. It is 26 degrees outside and the A/C hasn’t been used in a while. Empty house. Thoughts?

Beck 033.jpg

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What did it taste like?

Jeff is it possible to make the pictures smaller?? :twisted:

There is white crap on the ducts that should be cleaned by an HVAC contractor.

Sorry Brian. That is the size I have to reduce them to for my report software…but they look bigger on the reports, and I am too lazy to resize them again!!! Marc, it tasted kinda “squishy” and "green! Have you guys ever run in to this? (I did recommend cleaning by a HVAC tech).

sorry Jeff pictures to small

This any better?

No. Probably still tastes the same.

any excess moisture-- or humid in there

No excess moisture right now. 26 degrees outside, and the house has been vacant since this summer, so the A/C hasn’t been run in a while. Basement was bone dry today, and appears to have been for some time.

I would call it it out as unkown bio growth and suggest further exam, I dont see anymore in that picture growth I mean strange.

Yes, it looked strange to me also. It was only on the duct, the entire length of the basement. It did not spread to the wood anywhere, and it was in contact in several places.

It is absolutely not mildew. Mildew is a fungal parasitic disease the attacks living plants. We HI’s should erase the word “mildew” from our vocabulary as it is very rarely encountered on anything but living plants.

I doubt that it is mold because foil does not support mold growth. It is possibly mold if the surface is coated with alot of dust, sawdust, dryer lint, cooking grease, etc., but the photos suggest that is not the case.

I think it is probably aluminum oxide because of the basement atmosphere being potentially basic (ph) and in a closed, non-conditioned environment. Also, it could efflorescent accumulation due to ongoing condensation.

I have encountered 3 situations that can look very similar. The first one is mold growth on the ducts from excessive moisture. The second is when the foil covering gets saturated and moisture causes small blisters under the the clear UV covering of the foil. The third is from excessive moisture inside the duct causes the zinc covering to corrode and leach out through the insulation, almost looks like efflorescence.

It would be impossible to diagnose the problem without actually being there. If you are not sure what it is, recommend further evaluation by a qualified contractor or offer to take a sample of the substance and submit it to a lab if you are qualified.


I have also seen growth on left over bacterium. There is some very interesting research being done at Cornell that demonstrates communication between bacteria and fungi. Anyway, just thought I’d add this observation.