Not sure how it happened but my inclination is dry Rot. I don't see much of any mold or mildew around and the room does not feel damp or smells moldy so I'm really not sure if fungus is the cause. I'm trying to finish the space and frame a wall under this joist but I'm afraid that I won't be able to nail into it since it looks like shredded paper. I looked up some techniques to strengthen floor joists so my main ask is:
What is this? How does it happen? And how can I repair it DIY? (Some people online recommend consulting a structural engineer, but I'd rather not if possible) I was thinking of sistering the joists, but with it being a pocket joist I'm not sure if that's ideal.
Added a second picture for what I think is the dry Rot, but it looks small and not dangerous?
Shredded paper and little dried mud spots. That's old termite damage. I'm not sure how to fix your structural issue, but you'll need to see how much damage there actually is. Does the wood around that area feel solid or soft?
Still feels fairly solid, although near the shredding it kinda sounds hallow, I could be wrong through, I might not be hearing it right. The other joists don't have any damage whatsoever, they sound solid. This is a 50's house, so it feels like whatever damage happened it was years ago. I'm not sure where to go from here, sistering two joists sounds like it can help structurally, but I'm just not sure if that's the right move... Plus I have pocket joists so I'm not sure what kind of madness is involved taking the old one out and putting the new one in