On Episode 4 of Build Science™ 401, Matt Risinger & Steve Bazcek walk through the building envelope as a fully integrated system, emphasizing (you guessed it) that continuity of water, air, vapor, and thermal control layers is what ultimately determines durability and performance. Matt and Steve reiterate one of their most common warnings: water is the number one enemy, followed closely by air, and that most “vapor problems” are really failures in water or air management. Using a real slab-on-grade project, they break down foundation, wall, and sheathing details to show how integrated products like ZIP System sheathing can provide both water and air control while still allowing assemblies to dry. The discussion also highlights the importance of avoiding vapor sandwiches, designing for drying in both directions, and understanding diminishing returns when chasing the last BTU. The episode closes by reinforcing a practical building science mindset: get water management right first, maintain continuity across all assemblies, and balance performance gains against cost and complexity.