Is thermo col can be used as good residential under deck insulation?

Yes...Thermocol has best insulation properties, for better results you should go for a 14/15Kg m/3 density of thermocol. Underdeck insulation is applied with the help of G I Wires attached from wall to wall & should hold the thermocol sheets properly. Also hang wires from concrete ceiling & attach vicers at the end to hold the thermocol sheets. Fevicol SR grade is normally used to stick thermocol, but somehow the joints will get exposed. Try applying decorative wall paper with the help of fevicol. Hope this helps. :-)