Moisture in Assemblies: Diffusion, Convection and Leaks

A damp patch can result from leakage, moist airflow or diffusion-related accumulation. Repair starts by identifying the mechanism.

Diffusion

Water vapour passes through materials under the relevant conditions. Assessment needs layer properties and boundary conditions; it is not the same as air flowing through a hole.

Air transport

Airflow carries moisture through leaky connections. Finding the leakage path and restoring the air barrier can matter more than changing the main layer’s vapour properties.

Liquid water

Rain, faulty flashing, plumbing leaks and capillary uptake need their own investigation. A diffusion calculation does not assess these sources.

Accumulation and drying

Establish when wetting began, how it responds to rain or frost and whether the assembly can dry. Include initial construction moisture.

Common mistakes and analysis

Covering wet material without removing the cause risks recurrence. Combine inspection, measurement and an appropriate model for complex problems; a photograph alone does not establish the cause.

ZeroHouse assessment diagram; not a construction detail.

Project review matrix
Item	Parameter	Check / result
After rain	Possible rain leak	Inspect rain control
At joint	Possible air leak	Investigation
Within layers	Accumulation	Moisture assessment

What this means for your project

Before repair, collect a timeline, photographs and conditions associated with wetting. Do not label every moisture problem as dew point without investigation.

Sources and scope of review

Checked: 2026-09-09. Editorial guidance; not evidence of compliance for an individual building.