The Natural Philosopher formulated the question :
Not insulation. Thermal Mass.

Both help.

te huge lumps pf masonry in my chinmneys and the huge cast concrete ground
floor, all end up at the average tempearture beytween day and might. Around
26C

Likewise here, plus a few tons of stone.

By day, draw the curtains and shut the windows. at night reverse that./

Ideally, for cool in the home - when the outside is warmer, close
windows; when the outside is cooler, open windows. Problem is with
closed windows it can seem stuffy indoors in hot weather.