Are you sure all that humidity is coming from the outside? It looks like a really weird spot for that to happen
I did not post a pic of the whole facade, but to the right of the plant is an overhang that prevents water from getting on the facade. This section has no overhang so rainwater runs down the facade. A few years ago part of the facade was cracking and I re-rendered that part of the facade. After a rain, the new part of the facade remains bright in color which indicates water does not penetrate. But in the bad area, the gray surface becomes notably darker, which suggests water is penetrating.
Why can’t you access the upper floor? Is this an apartment building and someone else lives above you?
I have access to the whole house. The interior pics are of the top floor. What I meant by not accessing the top of the beam is just that the ceiling is in the way. I could remove the drywall on the ceiling corner to get access to more of the beam. I will first dig up the exterior facade and see if that exposes the top of the beam. If not, then I would remove a bit of the ceiling.
What’s in the black tube?
I’m not sure what you are referring to. My exteriour pic is terrible (bad camera). On the interior pic, there is a grey cable, probably a/c to the ceiling light. The blackness to the left of that is not a tube but simply a missing brick. It had plaster and insulation foam before I got to it. But now it is just a hole. If you mean the exterior, it’s just a terrible pic. Above the plant is a wood panel that is really warped from getting wet. I installed it new a few years ago but got something wrong. It may not have been treated wood, I don’t recall. I thought painting it would be sufficient but clearly not in the section that has no overhang.
Perhaps you have already checked, but I’d try to rule out any infiltration coming from above, otherwise it will keep happening and it could get worse.
I will know more after I dig up the facade. But note that the roof is right above and just to the right of the plant (off picture) is the downspout. It’s clear, but I suppose I have to wonder if it’s possible that water puddles at a bottleneck right where the downspout is.
I appreciate all the advice! I still have to dig up the exterior but there is no growth of any kind on the interior side of the beam. It’s drying out and getting less soft as it dries, but still soft enough that I can stick needles in it ~2cm. The bottom side is the softest. I used a syringe to squirt in some wood hardener.
I am tempted to use pro-grade hydro PVA and plasticizer in the exterior concrete then finish with a waterproofing clearcoat that claims to still allow breathing. I believe that will mitigate rain saturating the facade. But on the inside wall I am tempted to not even plaster and just leave the brick and wood exposed. I would then expect water to not penetrate from the outside, but if it does so anyway then at least it could breath on the inside. I can also monitor it that way.