How good is it?
The Passivhaus is so well insulated that body heat (70 watts) is just about enough to keep it warm. Add passive solar from big south facing windows and you don't need much else. Most passivhaus designs had perhaps too much of a good thing with the windows, and it became a bit of a problem in summer. Martin Holladay notes:
"Although the Passivhaus Institut recommends that window area and orientation be optimized for passive solar gain, the institute's engineers have concluded, based on computer modeling and field monitoring, that passive solar details are far less important than airtightness and insulation R-value."
More: Passivhaus for beginners
Credit: Passivhaus Institute