The IEC 60287 does not list materials for filler but states: depending on the filler material and its compaction, the thermal resistivity is in the range 6 to 13 K.m/W for cables with extruded insulation, 10 K.m/W is suggested for fibrous polypropylene.

Following values are taken from IEC 60287-2-1 Ed.2.0 for insulation or jacket with following additions:

Symbol
$\rho_{\mathrm{f}}$
Unit
K.m/W
Related
$M_{\mathrm{f}}$
$T_{\mathrm{2_{\mathrm{1}}}}$
$T_{\mathrm{2_{\mathrm{f}}}}$
Choices
MaterialValue
FPOC6.6
FPP10.0
PVC6.0
PE3.5
PR8.0
PT12.0
TY5.3
Jute6.0
tape6.0
Air50.0
Ot10.0