Building Envelope and Weatherproofing
The Four Control Layers
A strong candidate can describe the envelope as four functions: water control, air control, vapor control, and thermal control. Understand that water is managed by shedding, drainage, and drying, and that the drainage plane must lap shingle-style (upper over lower). Know where the air barrier and vapor retarder belong relative to insulation and climate. Recognize flashing at penetrations, roof-wall junctions, windows, and foundations.
Traps to Watch
The classic trap is placing a vapor retarder on the wrong side of the assembly, trapping moisture. Another is confusing an air barrier with a vapor retarder; they serve different purposes, though one product may do both. Questions may offer "caulk the crack" as a fix for bulk-water entry when the real need is flashing and drainage. Watch for reverse-lapped housewrap and missing kick-out flashings.
Worked Example
A window leaks at the sill during wind-driven rain. Rule of thumb: water must be directed out and down. The assembly needs a sill pan with a back dam and end dams, integrated with the drainage plane, plus an air seal at the interior. Simply sealing the exterior perimeter with sealant fails because it relies on a perfect seal and traps water behind it.
Key Points
- Manage water by shedding, draining, and drying.
- The drainage plane laps shingle-style, upper over lower.
- Vapor retarder location depends on climate and assembly.
- Air barrier and vapor retarder are distinct functions.
- Flashings at penetrations and transitions are critical.