Super foil v fibre insulation

ahfat41

My loft insulation needs topping up. Was thinking if super foil instead but do not know much about it. Can anybody advise and the cost to insulate. Thanks

grumpy_codger

Super foil - is it the one 30mm thick? 1 m²K/W R value?
It does reduce heat radiation, but 30mm cannot do much to stop heat loss by conduction.  
I think for mineral wool the current requirement is 25cm. And its R-value is about 2.25 AFAIK. So, if you need, say, 15cm extra of mineral wool, i.e. about 3.4 extra in terms of R-value, then it has to be more than 3 layers of the above 'super foil'. 
Don't trust my calculations - do you own.