You maybe stayed in that house in Rome but didn’t live in it. I grew up in a 100+ year old stone and mortar house with almost 1.5m thick walls and relatively good window insulation. The problem is the thermal capacity of those walls. When they heat up during a hot summer spell it’s like living in an oven. You cannot sleep most nights during the summer. Something similar happens in the winter too, if you go on a vacation. The house gets so cold you need couple of days to warm it up again. When my parents installed AC it was heaven during summer.

Glass windows can be surprisingly efficient insulators. Have you seen multilayered argon-filled glazing? Or vacuum glazing? The best are as efficient as a triple-brick wall.

> I stayed in a house in Rome that kept out the fierce summer heat, because of thick walls.

Doesn’t work when the overnight low is 20C

I would need a sweater with 20C...

you won't when the thick walls and all objects are radiating 30C heat at you

Most (not all) old euro cities don't get a good breeze unless they're built on the windward side of a hill because (generalizing here): the streets aren't a grid and everything is built to the same height.

plus the humidity is still there without aircon. 20C humid != 20C dry.