Garden office floor - 95mm timber - 100mm PIR

[Wockawocka]

Hi everyone,

I've a 7x3m concrete base that I need to frame and insulate but I'm seeing general pir sizes are 60, 80, 100mm but the timber merchant on supplies 95mm.

Should I be using 95mm timber and 75mm PIR with a gap underneath instead? I kinda wanted 100mm in there though.

Thanks!