Would it be ok to just have a 30mm air gap (rather than the standard 50mm) between the kingspan pir and the under side of the roof battons plus the extra bit of space (0mm - 10mm ) between the under side of the battons and the lime mortar underneath the slate. I dont have any felt. just the lime mortar which fills in the gaps between the slates. I am only insulating 3 meters of the bottom part of the roof as the top part is attic space. There is no air vents at eves so i will probably drill a few holes at the eves to allow some air flow to get through inbetween each rafter. Any input anyone? Does that sound ok to you?