Advice on insulating a loft with 65mm / 2.5inch joists

[mitch1000]

Hi,
Can anyone offer some advice on insulating our 150 year old house....Having just moved in and discovered it's not properly insulated.

The loft is completely lined with a paper-like material that has foil facing down onto the joists and the upper surface feels (and tears) like sandpaper. Between the joists and the ceiling is nothing; no insulation. It looks like the ceilings were replaced at some time with modern plasterboard, but no insulation was replaced. It's possible that there was some form of blown insulation in the loft and the paper material was to keep it in place between the joists.

The problem I am having is that the joists are only 65mm tall and spaced approximately every 300mm.

I could lay insulation at 90 degrees across the joists, but then there would be a void between the ceiling and the insulation, I can't find roll-insulation that is 60mm to lay between the joists; the lowest depth is 100mm. I am wondering whether I should use 50mm slab insulation between the joists and then lay roll-insulation across the joists. I also thought about buying 40mm strips of wood to increase the height of the joists, but this would be a lot of work and difficult to get through the loft hatch.

Thanks for any advice,

Mitch