@johannes_lehmann @Ruth_Mottram
My house in Cambridge was built with walls two bricks wide, in a two parallel, one across pattern. No cavity, and no insulation. Had coal-burning fireplaces at every room (unused but leaking cold air) and radiators heated form a gas furnace. Energy rating E.
We added 10 cm of a dense modern material to the walls, mostly wood pulp as far as I know, and changed the windows to double or triple glass panel. And replaced the heating with an air-source heat pump. Energy rating A+.
This should be done for easily more than half of all homes in the UK.
https://albert.rierol.net/tell/20221008_insulation.html
If, around 1900, all the effort, money, and thought that went into mining and transporting all that coal had gone into building houses with cavity walls and thicker windows, all of this mess would have been averted. Alas, a few fat cats wouldn't have become rich. Tragic. Time for the rich to pay it back.