Insulating loft rafters

I would use 100mm kingspan batons cut to a snug size and tap them in between rafters with a rubber mallet making them flush with the face of the rafters, this will leave a void between the slates/tiles/felt and insulation and the roof will still be able to breath and not sweat.

What depth are your rafters?? For storage only you can use the foil roll that can be bought from pretty much all builders merchants, you MUST keep an air gap between the underside of the roof tile and the insulation. All the best Mat

I would recommend 100mm rolls of earthwool insulation. It is a cost effective solution made from recycled glass you can install between your rafters then board over.