I'm guessing this is the sort of stuff: https://www.screwfix.com/p/ybs-airtec-reflective-double-insulation-25-x-1-2m/6520p
That has a thermal resistance of 1.5 W/m3K
100mm of fibre glass is a bout 2.85
This 40mm stuff suggests 2.5 https://www.screwfix.com/p/ybs-superquilt-multi-layer-reflective-foil-insulation-10-x-1-5m/68120 so a much better option for you.
Kingspan/Cellotex is better.
I decided to go Fiberglass – cost and easy (except I hate to handle the stuff)