Scope: This article supports product selection and technical discussion. It does not replace the project designer, local building code, approved construction details or product-specific declarations.
Liquid water and water vapor are different loads
A breathable membrane is designed to resist bulk liquid water while permitting a measured amount of water vapor to pass through. The result depends on membrane construction, test conditions and intact seams. It does not mean that open joints, punctures or reverse laps become waterproof.
The pressure difference drives diffusion
Water vapor moves in response to vapor-pressure differences across the assembly. The membrane can support outward drying only when the rest of the roof or wall also provides a viable drying path. Exterior cladding, ventilation cavities, interior humidity and insulation temperature all affect the result.
Use the membrane as one control layer
Place the exterior membrane so incidental water drains outward, then maintain continuity at overlaps, openings and penetrations. Do not use it as a substitute for the primary roof covering, flashing, drainage cavity or an interior vapor-control layer required by the project design.
Project checks
- Confirm the membrane position relative to insulation.
- Compare vapor data only under the same test method and conditions.
- Detail laps, openings and penetrations before installation.