sd-value

The sd-value expresses how resistant a building material is to water vapor, shown as the thickness of an equivalent layer of still air (in meters). It is calculated by multiplying the material’s µ-value (vapor resistance factor) by its thickness.

Formula:

sd=µ×dsd = µ \times d

sd=µ×d

where d = thickness of the material (in meters).

How to read it:

• An sd-value of 0.5 m means the material blocks vapor as much as half a meter of air would.
• A low sd-value (< 0.5 m) = vapor-open / breathable.
• A high sd-value (> 10 m) = vapor-closed / highly resistant.

Why it matters:

• Helps compare different materials regardless of their thickness.
• Essential for designing walls and roofs that avoid condensation.
• Used to decide whether a vapor barrier or vapor-open layer is needed in construction.

👉 In short:

• µ-value = property of the material itself.
• sd-value = property of the material in context (depends on thickness).

Example calculation of sd-value

Suppose you use wood wool insulation with:

• Thickness (d): 28 mm = 0.028 m
• µ-value: 5 (moderately vapor-open material)

Formula:

sd=µ×dsd = µ \times d

sd=µ×d

Calculation:

sd=5×0.028=0.14 msd = 5 \times 0.028 = 0.14 \, m

sd=5×0.028=0.14m

Result:

This layer has the same vapor resistance as 14 cm of still air.

👉 That means it is still quite vapor-open, allowing moisture to move through.