alt Hacker NewsIt's not so much the temperature per se, it's the fact modern houses are designed around AC and American houses in particular are basically giant wooden garden sheds.
In a traditional house with thick brick/stone walls and large roof overhangs you'd be uncomfortable but a lot less likely to die
It would be smart (assuming a properly insulated and air sealed house--which to your point is uncommon in the US) to build an insulated room with a dedicated hvac zone in the center of the basement or somewhere similar that is basically your "safe room" for such a situation. Would not take much to cool it and has the advantage of being maximally distant from the exterior walls to in case you are worried about radioactive fallout ;)