Over Deck Insulation versus Underdeck Insulation.Which one is better Cost wise as well as convenience in application?

My submission is, the underdeck insulation is any day beneficial in terms of convenience in application  as well as its cost effectiveness as detailed below:-

The Overdeck insulation Extruded Polystyrene Boards come in the sizes of (650mm X 1250mm). If it is required to put 100mm thickness, it  will be laid as two different layers 50mm each. Although XPS has far better compressive strength than thermocol, still utmost care is needed in their protection while doing screed/B.B.Coba over larger roof spans so that these boards don’t get punctured. It is hard to keep them intact due to falling of concrete, laying of brick bats &  labour movement over them.

This will also affect maintaining the perfect slopes towards khurrah positions. It is very much likely that there may be development of cracks in screed/B.B.Coba done over these loosely laid & dislodged boards, thus resulting in possible waterproofing failure.  

The underdeck insulation, otherwise, is an independent activity, that too is carried out as the last activity before handing over the property.  

 Moreover, the additional cost of overdeck insulation over underdeck insulation is as under:-

1. Additional Cost of XPS Board over under Deck insulation =Rs.850/- per Sq.m
2. 120 gsm geotextile as separation media                           =Rs.90/- per Sq.m
3. Wire mesh to protect Cracks development in Screed        =Rs. 150/- 

                                                                     Total     =Rs.1090/- per Sq.m.