Thickness of screen serving

The screen serving usually consists of water-blocking swelling tape.

Its thickness is added to the thickness of insulation to sheath $t_1$ which is used to calculate the thermal resistance between conductor and sheath. Therefore it has the same thermal resistivity as the insulation in the calculation. In reality, it usually has a higher value, but the error is small because the layer is thin with typical thickness of approx. 0.5 mm.