Moisture Management
Sources and paths
Moisture enters as bulk water, capillary suction, vapor diffusion, and air transport. Air transport is the dominant path in most assemblies because it moves large amounts of vapor quickly through leaks. Candidates must distinguish the four and know the control for each: drainage and flashing for bulk water, capillary breaks for suction, vapor retarders for diffusion, and air sealing for air transport.
The layers and their order
Know the function of water-resistive barrier, flashing, vapor retarder, and air barrier, and that they are not interchangeable. A vapor retarder belongs on the warm side of the assembly in cold climates; in mixed and hot-humid climates, Class II or III retarders and vapor-open assemblies are often preferred to allow drying. Traps include treating housewrap as a vapor barrier (it is generally vapor-open) and putting a strong vapor retarder on both sides of a wall, which traps moisture.
Wet basements, crawlspaces, and roofs
Understand that the ground under a slab or crawlspace is a major moisture source and that a sealed, insulated crawlspace with a ground cover changes the pressure and moisture dynamics. Know that roof assemblies need ventilation or a designed unvented assembly, and that ice dams come from warm roof decks plus snow, not from gutters alone.
Worked example
A homeowner reports condensation on the inside of window glass in winter. The glass is the coldest surface, so interior humidity is above the dew point of that surface. The fix is to reduce interior moisture (source control, ventilation) and improve window performance, not to add a vapor barrier to the wall.
Rule of thumb
When in doubt, let assemblies dry in at least one direction and keep bulk water out with flashing and drainage.