We've worked out why Karl and Nini's bedroom floods. We can chalk it up to the previous owners, again, just like all the other inanities we've found so far. In this case, they installed a french drain... right up against the foundation.
Roof run-off, channeled through downspouts, emptied into the french drain, which would quickly fill and slowly release water into the joint between the footing and the slab. A high-water mark on the footing, outside, showed quite clearly where the top of the slab was, inside. The low point of the drain was at the corner of the house (the sump pump, 10 feet away, was uphill by about a foot). This was hiding under a 12" cap of good Bay Clay, making the problem invisible.
Karl and I started, and then Jori continued, digging it all out. It goes down a good 3 feet. We'll pour a little concrete skirt to drive the water lens well below the foundation, and then replace the french drain to be between the two houses and slope towards the sump pump.
I also rerouted the downspouts. We clearly haven't finished this project, yet the nearly 2" of rain that just dumped on us didn't get into Karl and Nini's space, so we've already made an improvement.
Sustainable parts of this job are we'll be reusing the drain rock, so the embodied energy is very low, and we're working by hand, and we're preserving the habitability of an existing asset. We will have a concrete pour, so that's not as good, but I am sure any CO2 release from the manufacture of the concrete is offset handsomely by not having to run heating fans on a wet carpet three or four times a year.
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