In Ohio there is almost always problems with leaking in the basements or moisture build up.

I try to gently let tenants know that each fall/winter, in an really old graystone, there are probably going to be some occasional incursions.

There's a moisture meter that costs $40. 

The low volume / quiet fans have been useless for us, they just do not clear out the moisture. 

My home is only 1/2 mile from the water, thus it is spared the driving salt and rains, excessive moisture, and sand damage from homes directly on the water.

It could go tomorrow or it may last another 10 years4) Any idea what's causing the mold- what's the moisture source?

As far as leaks, a moisture meter will tell you if it’s an active leak.

Below are the details, please let me know if there are any bases left to be covered, additional questions I should have answered, etc: - 3 Bed, 1 Bath, 1106 SF- $695/month, current rent value is $1000-$1100/mo - Tenant pays on time - There's a mild subfloor/foundation issue in the corner of a room (realtors guess is that it could be a failed rim joist from moisture - is this an expensive fix?) 

There was no fan in the bathroom to remove the moisture.

Below are the details, please let me know if there are any bases left to be covered, additional questions I should have answered, etc: - 3 Bed, 1 Bath, 1106 SF- $695/month, current rent value is $1000-$1100/mo - Tenant pays on time - There's a mild subfloor/foundation issue in the corner of a room (realtors guess is that it could be a failed rim joist from moisture - is this an expensive fix?)