Loft conversion insulation

Mulling over the possibility of doing a loft conversion. Very early days and probably not till next spring.

House built in 1925 with a plain tile roof in good condition. A dormer will be on the back, but I was wondering if I can simply put the required amount of PIR in between the rafters from inside as the roof is original and has no felt/breathable membrane under the tiles. Don’t fancy stripping a good roof. Also, I’m aware for flat roofs, 120mm is required, what’s it for lofts? What about air gap or can I simply push PIR up to the battens?

cheers

I put a 2x1 batton on my rafters to space them out because I got a load of 100mm PIR on the cheap.

I think they originally spec’d 75mm PIR between the rafters with a 50mm air gap above, then a full 50mm PIR under the lot followed by plasterboard.

Cheers. Basically 120-125mm total then.