I live in a relatively new building in NL, where many of these configurations are applied and on its own it won’t go lower than 21C in winter and 25C in summer. Thick walls, thick anhydrous cement floor layer for insulation (although I think it’s also meant to give the option to implement floor heating), _external_ blinds on the South side.

I simply can’t justify a smart thermostat/radiator valve build, despite my inside nerd constantly whispering I should.

Only downside is the heat exchanger air recirculator: it dries the air terribly in winter. You need a humidifier to avoid getting parched skin and mucosa.

This is the answer! Even more important than insulation is air sealing, so you keep the conditioned air inside.

I've learned an enormous amount about building efficiency from

https://www.greenbuildingadvisor.com/ and https://www.energyvanguard.com/