Black residue outside gable vent

Hey all, what do you think the black residue is collecting outside this gable vent? Thanks in advance for the help.

Warm moist air exiting the louver condensing on the siding turning to black mold.


What did it look like inside the attic? Did the house have an attic fan?

1 attic fan, no readily attic access.

Attic fan blowing loose insulation out of the gable vents??

That was my suspicion. Any way this could be mold build up like the prior post said?

What direction does that vent face. Mold growth in direct sunlight greatly reduced.

When you speak of attic vent, are we talking about a powered roof vent or a whole house fan?

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I agree with this - I see this all the time in Oregon. On the roof you can always see which vent the bath fans terminate at since it’s got black mold crap all around it (older installs when terminating fans at vents was common). I’d bet there’s an exhaust fan of some type terminating at the gable vent.


Gable vent shown with discoloration.

East side of house

There is a second floor bathroom, so it may be the bathroom exhaust

Need more info.
What venting system did the attic have? Direction of airflow varies on which.
Automatic or Manual fan?
Gable vent and fan?
Gable, ridge, and fan?
Gable, soffit, and fan?
Gable, soffit, ridge, and fan?

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Gable vents, automatic fan and soffit. Wind in this area usually blows from west to east.

Is the fan located in the gable vent, or roof top?

What is your area??

It is located near the cent I believe, the home owner is going to send me more pictures. This home is in mid Michigan.

Typical of gable vents. Same thing happened to mine (but not that extreme) and had it professionally pressure washed last summer.
Common to see around this area after a few years of neglect washing the vinyl.


Would be good to see, but I would defer to a ventilation expert.
So many questions about the system that the homeowner prolly has no clue.
Such as… Does it incorporate a Humidistat?
Is the system operating properly?
All of which are outside the scope of a home inspection.

Something to be concerned about or just regular maintenance of cleaning?