Thickness of screen bedding

The screen bedding usually consists of semi-conducting 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 less than 0.5 mm.