Celotex / Kingspan etc vs Glass Fibre Insulation in a Loft

wing.nut007

I am looking for an answer if laymans terms as to how much more efficient PIR Board insulation (specifically) ecotherm is when compared to glass wool.

Reason for my quesry is that I am insulating my loft and to date have fitted 70mm ecotherm between the roof joists. In the middle section of the roof I am looking to place further insualtion over the top of the joists and to then board to create a usable storage area. The outer areas I am looking to top up with fibre glass quilt.

I am aware I would need around 270mm to meet energy target but I am unsure and have had no luck finding out what contribution my 70mm of ecotherm makes towards this. My local building inspector quoted that 100mm of PIR insualtion is equivalent to 300mm of glass fibre but this does'nt sound right to me. I have searched the web but all I keep finding are U values which I am struggling to understand so I'm hoping someone here may be able to help me.

Many thanks.