The first episode of Build Science™ 401 frames the building envelope as one continuous, integrated system where air, water, vapor, and thermal layers must stay aligned—especially at foundations, penetrations, and transitions. Using a real-world ICF foundation, Matt Risinger and Steve Baczek explain how built-in insulation, rebar, keyways, and monolithic pours work together, alongside smart water management strategies like bagged footings, insulated gravel under slabs, perimeter drains, and ICF-compatible waterproofing. They bust common below-grade myths, showing through blower door testing that the ground can effectively act as an air barrier and that overcomplicating under-slab air sealing often doesn’t improve performance. The discussion also clarifies vapor barriers versus vapor retarders, concrete drying behavior, and risk-based decision making. The core message: details vary, but physics doesn’t—durability and performance come from continuity, compatibility, and assemblies that do multiple jobs well.