knocking down to build new is pretty unsustainable as well as unnaffordable.

can easily get underfloor and roofing insulation done, also walls can be injected with air foam. so its completely untrue that you cant bring an old home up to a good standard. average 3 bed house would probably cost around $6000 to completely insulate, add a few more for a heat pump and for 8 grand youre sorted. in terms of true energy efficiency retro fitting an old house by far outperforms demolition and rebuild when you take into account whole life and embodied energy etc.
Id argue you can make an old house as comfortable as a new house