A Recipe for Disaster: Gaps Behind the Bookcases

Because there are no straight lines in the house, and because the bookcases were level, both bookcases had gaps underneath where the floor sloped under them.  Before installing the bookcases, the workers had removed the baseboards.  Unfortunately, there was nothing behind the baseboards — just a hole through which you could look and see the ground under the house.  For some reason I took photos and didn't tell them to close these up --- I just assumed they would.  Here is the gap on the west wall:

 And here is the gap on the east wall:

When I used a flash, you could see the ground below the house!

They did not do anything to close this up.  They just built the bookcases up against these openings.  So it looked like the opening was closed, but because there were openings under the bookcases and cracks between the boards comprising the cabinets, outside air flowed through these openings, into the house, and into the cabinets at the base of each bookcase.  This caused hot, humid air to sit in the closed cabinets which resulted in........MOLD.  I didn’t know this until much later.  We ended up hiring our initial contractors to go under the house and cover up the openings, and Jamie caulked the inside of the cabinets.  This helped the problem go away, but it still gets humid in there.