On the Passive House in the Real World (with Mike Duclos) discussion board… 
Takeaways:

Homeowners looking to incorporate Passive House principles in existing buildings should consider getting a home energy audit. Still, after a home is built, serious energy reductions (say 50% to 90%) become very expensive.
Attic insulation adjustments and window treatments offer more immediate fixes toward energy efficiency.
Electricity is typically our dirtiest fuel, […]