Spent 2 years trying to figure out why my house had high humidity and occasional mold outbreaks on furniture and leather shoes. Bought a dehumidifier and it helped, but couldn't keep up. Finally bought a $6 hygrometer from Lowes and started placing it around the house. This revealed the humidity was highest in my bedroom, also the area with the most mold issues. By placing the hygrometer in different areas of the room, I found the humidity was highest - over 80% - at floor level right next to one wall on the North side of the house.
I knew from previous experience crawling under the house that this particular wall had been worked on. I could see some minor subfloor rot and the previous owner had shoved a couple of 2x10s on top of the sill plate for extra support of the subfloor. I figured the rotted subfloor was porous enough that a narrow stream of crawl space air was getting through under the baseboard and quarter round. When we ran the bathroom fan and put the house in a vacuum state, cool, humid air would suck in through these small gaps and that was enough to humidify the whole room near the floor.
So I crawled under the house and poked around behind the extra 2x10's. It was mush. Fock... The extra 2x10s weren't just propping up rotten subfloor planks, they were hiding a bigger mess!
I pulled out the newer boards and realized everything behind and below them was rotten. The sill plate. The band joist, and about a half-foot of the subfloor was all stuff I could pull apart by hand, and so I did. The wall's sole plate looked OK, but I could see the bottom side of my hardwood floors from under the house, meaning they were only supported by their own tongue and grooves. There was no joist - only spongy crumbles of what was once wood, and a few sections I had to remove with a reciprocating saw. Soon I was looking at my brick facade from the crawl space, which is not supposed to happen, but at least the demolition was easy.
The wall was completely unsupported, and had been unsupported for an unknown number of years. Apparently, the diagonal bracing in the walls was so strong that the room did not collapse because the rigid walls were suspending everything like a cantilever, even the floor. There were not even any wall cracks and no signs of sagging underneath either! This situation extends for 4' on one wall of the room, and at least 8' on another. I say at least because there is another set of extra 2x10s that I dare not remove until the section with the most issues gets some support. It could be fine, but I doubt it. So probably 20' of wall unsupported. :)
Here is a diagram of the damage (gray spray paint denotes absolutely rotten wood) and some of my first couple of pics from under the house.
I'm going to DIY this thing. I have a plan to repair it from underneath without removing the hardwoods. The plan involves replacing all rotten wood back to something like the original design. I will use a skill saw upside down to cut a straight line in the subfloor and remove all rotten parts. Then I'll attach a 1x12 from underneath. The new subfloor board will be held in place by the new band joist and brackets that will go between the new band joist and the adjacent original joist. I'll also be using lots of screws and liquid nails. I think the hard part will be getting the new band joist and sill plate wedged in place. Because I can't nail from the outside, I'll use metal brackets and screws to tie everything together nice and tight from the crawl space.
Will post updates as I get the chance. Should take me another week (INFAMOUS LAST WORDS, LOL).
Time invested: ~6h so far