Not insulation. In fact we have very little. But a few things that keep us relatively cool:
- Our east facing front is shaded by large trees on the Avenue. Just like a Provençal village square
- the second floor closest to the attic and roof space is not used - just spare rooms and studio. So the real heat tends to gather up there and in the attic
- Roof surface area is pretty small compared with house volume, as it’s a tall thin townhouse
I hope that future architects take into account summer cooling in design. There’s plenty of architecture focusing on winter insulation but I’ve found most modern houses and flats get unbearably hot in summer, indeed the new builds near the river in my area are too hot even in spring and autumn because they all have huge solar gain windows and low ceilings.
FWIW, I've been cultivating a large viburnum in the front garden... it's around 12ft tall now and blocks the sun from the dining room (which is where I'm writing this - the ancient thermostat reckons it's 25C in here, so is likely to be the coolest room in the house.)
As for roof space, that's the perils of loft conversions (done to my bungalow in the 70s, before we moved in)... all that heat gets trapped in the new bedrooms instead. Indeed, I remember as a youngster finding out that if I stood on the bed the air near the ceiling was really warm compared to bed-level.
Hopefully changes will be made to reflect the increasing frequency of noteworthy heat in the UK. I note that the new gov't planning changes, while bad news for the countryside, do contain one good thing - all new streets to be treelined. That simple change should cool things somewhat, as my viburnum and your avenue trees do.
Just hit 31.0C upstairs as I was writing this... and it's fallen (quite rapidly) to 28.5C outside.