Services layer in timber stud/timber frame structures

It may be a good idea to provide a services layer about 3 to 5 cm thick on the inside. The advantage of this is that lines for building services can be installed in this layer with no need to penetrate the airtight layer, which usually rests on the interior cladding of the main load-bearing structure; it can be installed and checked before work starts on installing the building services. The insulation of the services layer is on the inside of the airtight layer.

What is an airtight layer?

The airtight layer is a continuous layer in a building which is located at the inside of the external building components. The airtight layer surrounds the building without interruption. It can be drawn without interruption into a cross-section with a pen ("pen rule"). It surrounds the heated building volume.

Continuous airtight layer of a building (source: Schulze Darup)

In brick buildings the airtight layer is formed by the interior plaster, in timber buildings it is implemented by foils, vapour barriers or vapour seals.