Well, Maximius, yet trying to answer your discussion about why were the insulae upper floors the worst to live in, I'd just add the temperature problem.

I doubt there would be any ceiling layers under the supporting wood structure and clay roof itself. As it keeps happening in many houses in warm countries, those upper floors are terribly warm in the summer, as the sun heat is transmitted inside the chamber by irradiation from the roof. The closer the floor is to the ground, the cooler it can be in the summer.*

In the winter, any gap in the wooden structure and clay roof would allow the cold wind to come in, and thus not maintaining the heat inside.

Besides, pigeons were not a good company, and I imagine there might have been quite an amount of them living on the rooftops and niches of the insulae. Perhaps their presence and excrements could easily contaminate the place with parasites... who knows?

No wonder why they could have been the cheaper to rent!

* Note: Here goes a clever way to live in a cool domus in Northern Africa summer - one could build a second peristylum and rooms under the domus floor, thus having quite a cool place to stay in the warm days!