Check out the RB's label for black bars in factories: "Factory Interior Walls." Seems a good enough reason to call the Rowhouse black bars "Rowhouse interior walls" particularly when the two are stated to be "similar." O5.3.
Factory Interior Walls may "only" be crossed "via bypass (as per B23.71)" along an exterior vertex or via a Breach. O5.32. O5.34 also says that rubble has no effect on Factory Interior Walls unless there is rubble on both sides, as with Rowhouse black bars. So in the context of Factory Interior Walls Klas' answer cannot be right even though we have the right to expect the answers to be similar.
I can certainly live with any answer from Klas: I just would like the luxury of being able to defend it against those who won't accept "Klas Sez" (heretics, I understand, but they do exist). So, it seems to me a Q&A is warranted. Perry is a big boy: if he thinks it silly he can ignore it.
Here is what I would ask:
A23.71
Q. One hex of a multi-hex Rowhouse building is rubbled (at all levels). An Infantry unit then occupies the rubbled hex. May the unit (assuming sufficient remaining MF) then cross the black bar of that hex to enter an adjoining Rowhouse building hex, not rubbled to any extent, at the usual cost of 2 MF? May it "bypass" the black bar to enter an adjoining building hex at cost of 3 MF? And if instead the unit were to enter first the still extent, adjoining Rowhouse building hex, could it bypass the black bar to reach the rubbled hex and if so, at what MF cost?
Proposed A. No, the black bar still exists and therefore may not be crossed directly at the usual MF costs. No, because Rowhouse "bypass" is only available between to adjoining Rowhouse building hexes.
Here is a Gamesquad thread on the topic.
http://www.gamesquad.com/forums/index.php?threads/bypass-rubble-and-rowhouses.146933.
Your question ecz, you want to ask? Happy to have you claim this text as your own if you like. But give me a few more minutes: my searchable "Perry Sez" is a tad out of date so I should check the latest first.