The weatherproofing stack from outside in
A building envelope protects interior space from weather. The layers form a defensive system: cladding (exterior facing material) sheds water, sheathing provides structural rigidity and secondary water management, insulation reduces heat transfer, vapor barrier prevents moisture infiltration, and drywall seals the interior.
Cladding (brick, vinyl, metal) is the sacrificial layer exposed to rain, wind, and UV. Sheathing (plywood or oriented-strand board) sits behind cladding and provides drainage plane (a slightly gapped layer where water runs down and out, not into the building). Insulation reduces thermal bridging and air leakage. Vapor barrier controls moisture direction.
Installation sequencing and failure modes
The layers must be installed in correct sequence to function properly. Water resistance comes from overlap and drainage, not waterproof sealing (sealants fail eventually; drainage lasts). Insulation placement (inside or outside sheathing) affects thermal performance and dew-point location, which determines where condensation forms.
Common failures occur when the vapor barrier is placed wrong-side (trapping moisture inside), insulation is compressed (losing R-value), or the drainage plane is blocked (water finds path into structure). Older homes without proper sheathing or with single-layer walls lack the redundancy modern codes require. Understanding envelope layers helps diagnose why homes develop water damage and why envelope repairs require coordinated work across all layers, not surface fixes alone.