The sd value is a central concept in modern construction - and yet many people do not recognise it. It describes the resistance of a building component to the passage of water vapour and is therefore crucial for the planning of a functional and durable building envelope. If you want to avoid building faults such as mould or structural damage, you should know the sd value and its meaning precisely.

When constructing or renovating a building, protection against moisture plays a key role. This is where the so-called sd value comes into play - a parameter that indicates the extent to which a material can Water vapour passage hindered. But what exactly does this mean and why is this value so important for the building envelope?

What is the sd value?

Also known as the water vapour diffusion equivalent air layer thickness - is given in metres (m) and expresses how thick a stationary air layer would have to be in order to offer the same resistance to water vapour as the respective building component. The higher the value, the more impermeable a material is to water vapour. An sd value of 1 m therefore means that the material offers the same resistance as a 1 m thick layer of air.

It is calculated from the material thickness (d in metres) multiplied by the µ value (pronounced: "Müh value"), a material-specific, dimensionless diffusion resistance number. The µ-value indicates how much heavier water vapour can diffuse through a material compared to air. The higher the µ-value, the more difficult it is for water vapour to migrate through the material.

The migration of water vapour

Water vapour always moves by diffusion from the area of higher vapour pressure concentration to the area of lower concentration - usually from the inside to the outside. An unfavourable choice of material can lead to the vapour condensing in the component layers, which causes mould growth and structural damage in the long term.

To prevent this, the sd value must be thought through layer by layer. Materials with a high sd value generally belong on the inside to make it difficult for vapour to penetrate. Towards the outside, the layers should always be more open to diffusion so that moisture can escape.

Building materials can be divided into five categories according to their resistance to water vapour diffusion:

Vapour open sd value 0.0m to 0.5m

Vapour barrier 0.5m to 10m

Vapour retardant 10m to 100m

Vapour barrier 100m to 1500m

Vapour-tight from 1500m

Why is the sd value so important?

A faulty sd value structure in the building envelope can lead to Condensation inside the construction. Especially in winter, when the temperature differences are large, water vapour can condense within a layer. This not only jeopardises the building fabric, but also the energy efficiency and quality of living.

Conclusion

The sd value is an indispensable indicator for planning moisture-proof building envelopes. Only those who know the diffusion capacity of materials and combine them sensibly can avoid structural damage and ensure a healthy indoor climate in the long term. Especially in times of energy-efficient construction, a well thought-out sd value structure is indispensable.