Vapour permeability, vapour control and airtightness are related but different properties. A layer can allow vapour diffusion while acting as an air barrier.
Terms
Vapour control limits diffusion; an air barrier limits bulk airflow through gaps and layers. Do not select a membrane solely by a generic product name.
Material properties
Relate μ and Sd to the specific product, thickness and test method. This guide does not assign universal numerical ranges to materials or variable membranes.
Layer position
Position depends on climate, indoor conditions and the complete assembly. Drying needs and possible reverse moisture transport may not fit a single-season rule.
Connections and risks
Even a suitable material needs sealed joints and penetrations for airtightness. Manage liquid water separately; vapour control is not universal waterproofing.
Common mistakes
Uncoordinated low-permeance layers on both sides can restrict drying. Assess unusual assemblies rather than copying an unrelated detail.
| Item | Parameter | Check / result |
|---|---|---|
| Vapour | Diffusion | Layer properties |
| Air | Gaps | Continuity |
| Water | Rain, leaks | Drainage |
What this means for your project
Request the membrane data, placement rationale and connection details. Do not approve substitution without checking the complete assembly.
Sources and scope of review
- ISO 13788:2012 — Surface humidity and interstitial condensation
- PNNL / Building America — Moisture control for residential buildings
Related reading
- The Thermal Envelope: How to Define It Correctly
- Thermal Bridges: Where They Occur and Why They Matter
- Airtightness: What It Is and Why It Matters