µ-value

The µ-value is a measure of how resistant a material is to water vapor passing through it. It compares the material’s resistance to vapor with the resistance of air.

• A µ-value of 1 means the material lets vapor through just as easily as air.
• A high µ-value (e.g. 50 or 100) means the material is very resistant — almost vapor-closed.
• A low µ-value (close to 1) means the material is vapor-open and breathable.

Why it matters:

• Helps you understand whether a wall or roof can let moisture escape.
• Important when choosing natural insulation (like wool, hemp, or wood fiber) versus synthetic insulation (like PIR or EPS).
• Using the right combination of µ-values in different layers prevents condensation and damage.