A quick Google came up with this https://www.boards.ie/b/thread/2057334577
Is it ok before you cover it up.
With that type of non breathable membrane it may be best to have a vented gap between the insulation and the membrane. 50mm is often recommended, but 25mm minimum should be ok. But this relies on having continuous ventilation along the eaves and ridge.
Otherwise, again you could full fill the roof with insulation tight up to the membrane - and in this case no gaps anywhere are even more important. The concept is to stop any moisture getting into the roof via damp air, and condensing on the cold membrane.
In both cases tap all joints and across the rafters.
Good find, thanks! I googled the full "minitex 20 P" and that didn't come up...
Air gap then, or it will end up like that! That's why I'm asking so many questions, insulation is so confusing! Even builders I had out to quote for it said different things, that's why I'm looking to do it myself now... so celotex between the rafters with a 50mm air gap and taping required.
The roof is sound, I've lived here four years and the garage was built in the late 90's and apart from a guttering issue, everything is fine.