A rigorous German standard for insulating homes is so efficient that the house can be heated with a hair dryer.
Of course, all of this makes the homes difficult (and expensive) to build. The hardest part is making the homes nearly airtight. The foundation walls and ceiling all have to seal together perfectly. In effect, a Passivhaus home could almost exist on the surface of Mars. Making them air-tight also means they need special ventilators that pump in fresh air without pumping out the home’s heat