Fixing Celotex...

[HammieUK]

I'm nearing the stage in my loft conversion where I need to fit insulation, I've got a huge stack of 70mm thick Celotex, I need 2 layers of this across the entire roof space. The first layer is straight forwards to fix, just screw through into the rafters, but I'm unsure what to do with the second layer. I could just wedge it behind the stud work timber, which itself is nailed to the purlins, and should give a fairly tight fit, does this sound like a good idea? Or any alternative ways?

Thanks