It’s not the insulation that’s the problem, it’s the lack of shading. Modern building regs basically demanded in all but writing huge south facing windows, turning any house into a greenhouse. Old houses have overhangs, shutters, awnings etc. Especially overhangs were excellent because it blocked out the high summer sun without blocking the light and let in the low winter sun. But that wasn’t sexy so all new houses have done away with it and the new housing regs didn’t have any effective regs to ensure summer comfort.