But don't underestimate the effect of good insulation (including windows). When the temperature hit the upper 90's here, I didn't need any air conditioning at all -- just turned on a window fan after dark to bring in cool air, then turned it off in the morning and closed the windows, pulled down the inside sunshades and closed the curtains in rooms facing the sun.

The upstairs gets a little warm during the day, but is tolerable by bedtime since the outside temperature has cooled by then. The downstairs stays comfortable all day long.

It helps that this is an attached condo with neighbors to each side, so there's effectively no heat coming in from the sides, just the roof, front and back of the condo.

Insulation alone is not sufficient. During a heat wave, after 48 to 72 hours, the heat will go through the wall, eventually.

The time lag of the building material and the decrement factor are also very important. Tge time lag should ideally be close to 10-12 hours in order to dampen day/night variations in temperature.

https://www.new-learn.info/packages/clear/thermal/buildings/...

This is why most of the time passive solar involves carefully planning before you build. Otherwise the best you can do is sunshades, insulation, landscaping and white roofs.

We had 90+ degree days for 3 days in a row (and upper 80's before/after that)

Night time temperatures dropped into the 60's, so by turning on the fan at night (which blew air out and upstairs window, bringing in the cool air in throughout the house), I think we would have been fine even in a longer heat wave. By morning, it was cool enough to need a blanket in bed.

If temps didn't drop so significantly at night, then we may have had more problems.

The house is standard wood-frame construction, so doesn't have a lot of thermal mass, though it is on a concrete slab, so that may have kept the downstairs cooler.

Thermal wool has about 6 hours time lag and almost no thermal mass. The downstairs doesn't have a roof over it, so that's probably why. Typically attics/penthouses are the worst because they're sun heats them up through the roof all day long. So, white roofs, little to no thermal mass, ventilation is a solution in your area. Concrete also heats up badly and then releases the heat if it's exposed to sunshine.

Use of insulation and pulling down the sunshades and closing the curtains in rooms facing the sun is passive solar, in essence.