There are many resources available for those interested in learning more about energy efficient retrofitting and the Passive House Standard. You‘ll find serveral below – from Passive House Institute research, whether in report format or as part of the Passipedia online resource as well as from from previous initiatives such as EuroPHit, 3encult and Sinfonia.
Energy Efficiency Solutions for Historic Buildings (Handbook)
3encult.eu (also featuring case studies)
Retrofit with Passive House components | Download
Retrofit Handbook: Retrofitting with Passive House Components | Download (in German)
Research Group Volume 24: 2003 | Refurbishment with Passive House components
Research Group Volume 30: 2004 | Ventilation in refurbishments
Research Group Volume 32: 2005 | Factor 4 reduction for sensitive retrofits: Passive House components + interior insulation
Research Group Volume 39: 2009 | Step-by-step refurbishment with Passive House components
Research Group Volume 48: 2012 Using Passive House technology for retrofitting non-residential buildings
EnerPHit-Planerhandbuch – Altbauen mit Passivhauskomponenten (only in DE, 59€)