Episode 5 of Build Science™ 401 episode walks through a slab-on-grade retrofit detail to show how air, water, vapor, and thermal layers must work together as one continuous system for durability and comfort. Matt Risinger and Steve Baczek emphasize managing water and air first—using integrated WRBs, vented rainscreens, elastic sealants, and continuous insulation—so assemblies can drain, vent, and dry instead of trapping moisture. They explain how insulating the slab from above pushes it outside the thermal boundary, dramatically improving comfort while remaining resilient even in the event of leaks. Matt and Steve repeatedly stress that perfection isn’t the goal: small thermal bridges are acceptable when weighed against simplicity and long-term durability. Remember, educated builders know where to stop, prioritize continuity and drying potential, and design details that are robust, forgiving, and practical in the real world.