At least I wasn't in a crawl space on this job, and it really didn't involve a floor. Instead it was cypress board ceiling in a beach house. And the complaint was buckling of the boards. So maybe we can call it an inverted floor.
This beach house is several years old. The owners had hired an interior decorator to help get it in shape to sell. Among many other upgrades, the interior decorator recommended painting all this natural cypress a nice white color. Then brown gaps showed up between the cypress boards. Hmmm. So they caulked the joints between boards, and painted everything again. Then boards started buckling. Hmmm again.
I noticed that some weatherization and humidity control had been done to the house. They had sealed plumbing penetrations in the floor and spray-foamed the roof. But they hadn't made the attics airtight. So the attics were still vented, with warm, humid coastal air. This air contacted the backside of the cypress ceiling and caused the boards to expand and buckle. Prior to the upgrade work, the ceiling breathed much better. Excess moisture could move inward very easy, and house air likely moved out through the joints between the ceiling boards at some times. Both of these would help keep the boards more stable. But painting and caulking, and even the spray foam, changed things enough to result in the buckling. The solution was to finish sealing the attic, and that was going to be difficult because of the height and layout of the house.
The funniest thing about this house was the master bathroom on the third/top floor. It was basically a sunroom just above treetop level, with nothing but ocean view. Getting started each morning would be great, but it would be difficult to leave the view to go to work.
