Build Science™ 401 Episode 9 uses an early Passive House double-stud wall as a retrospective case study to explain how the four control layers must work together as one integrated system. Matt Risinger and Steve Baczek show how ZIP System sheathing was leveraged to do multiple jobs—water, air, and structure—while redundant sealing, staged insulation, and careful blower-door testing were used to confidently achieve the 0.6 ACH50 requirement. They demystify vapor control by focusing on whole-assembly drying potential rather than individual material perm ratings, explaining why no interior vapor retarder was used and why the wall has performed well for more than 15 years. The discussion contrasts double-stud walls with continuous exterior insulation, weighing performance, cost, constructability, and lost floor area, while emphasizing experience, judgment, and “builder wisdom” over blind reliance on modeling.