Mold in the corner

I have been in a few newer homes (10 years or less) and have observed mold and subsequent black staining in the top 2 feet of a poured basement corner.

I am trying to figure out why.

Inconsistancies between the houses are:
Parially finished and unfinished basements.
Direction of corners
sump pump no sump pump.
occupied unoccupied.
no discernable outside leaks

The only logical explanation I can come up with is high humidity at some point and the cold foundation wall lowering the dewpoint.

Am I on track or is there another possible reason?

I live in the midwest.

Did these homes have bathrooms in the basements, typically with a shower? How about did the laundry areas have ventilation fans? As more and more homes get built air tight, the more we need ventilation to eradicate the moisture buildup. Added to the cold walls as you indicated, it’s a recipe for disaster. Don’t know about in your area, but in mine Air Exchangers are mandatory by state law, and most are left running year round.

Thanks Jeffery, These homes did not have air exchange systems. One area was directly above an open sump pump. The others, no obvious moisture creators anywhere close.

Were the pit’s “bone dry” or was wetness appearant? That could be your source.

Air Exchangers have been required here since April 2000, but many counties didn’t require them until 2006. If you are unsure what your area requires, it is worth looking into.