The core idea
A wet mark can come from liquid water, vapor transport or both; the repair path depends on which transport mode is active.
How the mechanism works
Liquid water follows gravity, pressure and openings, while vapor moves through diffusion and air leakage before condensing on colder layers.
When it becomes relevant
Typical trigger: Rain, plumbing use, indoor humidity or a rapid temperature change.
Evidence that supports this interpretation
- Visible flow lines or a sharp event link
- A repeatable humidity and surface-temperature relationship
- A directional path through a joint or cavity
Evidence that weakens it
- No change during the suspected trigger
- The mark is isolated between glazing panes
- Readings remain stable across comparable conditions
What this concept cannot prove alone
A single observation, image or meter reading cannot confirm the complete failure pathway. Use this concept to form better questions, compare alternatives and decide which safe observation would be most informative next.
Safety and professional boundary
Opening concealed construction or applying membranes should wait until the transport mode and drying direction are understood.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
