Who to contact to identify source of and address excessive humidity in house?
I had a home in Ohio with a basement that had high humidity a large dehumidifier running 24-7 pulled an amazing amount of moisture out of the air. Late in the summer we epoxy painted the floor with a 2 part epoxy and this alone greatly reduced the moisture in the house. 

We had taped plastic down in 5 places in the basement for 24 hours  to see if there were places that were worse (the front of the house was very wet, we had to wait until summer for the moisture to be low enough to paint (according to the directions) after that we did not need the big dehumidifier coating the floor but we still did use a much smaller model , I have coated quite a few basement floors since then all with similar results so you may give this a try.