If you're reroofing then I assume you will need Building Control approval and possibly Listed Building Consent.
If BC are involved then there are minimum thermal requirements you'll be expected to meet and to demonstrate you are meeting.
Our SLATE roof was insulated between and over the rafters. Ecotherm (solid foam board) was slotted between the rafters, scribed to fit the edges and then expanding foamed into a tight grip. The boards of Ecothem were laid at right angles to those fixed with long screw like nails called Helifix, the edges taped with aluminium tape.
Then battens were fixed over the boards in line with the rafters to allow roofing felt to be draped over the top.
Then the roof was counter battened over the felt to hang the tiles.
I would imagine that something similar can be done with a thatched roof.
If you need to keep breathability a.k.a. permiability for moisture then you can't use solid foam and you'll probably need to use a 'wool'.
Be aware that to meet regs you will probably need around a foot of 'wool' (sheeps, rock etc) or about 100mm of foam board. That will undoubtibly change the roof line. Guessing with thatch you're unlikely to have facias & gutters to worry about but you'll need to look at how that works on the gable ends and where the roof meets any other parts of the builing.
Kingspan, Ecotherm, Celotex etc. all have detailed technical documentation on how to use their products and meet the regs. If they don't have something on their web sites then you should be able to get in touch with them to find out more about their recommendations.
You need to make yourself a mini expert on this as BC and builders are generally not that clued up on how to meet the regulations. Some of my insulation elsewhere was installed so poorly that it has little or no thermal value.
If BC are involved then there are minimum thermal requirements you'll be expected to meet and to demonstrate you are meeting.
Our SLATE roof was insulated between and over the rafters. Ecotherm (solid foam board) was slotted between the rafters, scribed to fit the edges and then expanding foamed into a tight grip. The boards of Ecothem were laid at right angles to those fixed with long screw like nails called Helifix, the edges taped with aluminium tape.
Then battens were fixed over the boards in line with the rafters to allow roofing felt to be draped over the top.
Then the roof was counter battened over the felt to hang the tiles.
I would imagine that something similar can be done with a thatched roof.
If you need to keep breathability a.k.a. permiability for moisture then you can't use solid foam and you'll probably need to use a 'wool'.
Be aware that to meet regs you will probably need around a foot of 'wool' (sheeps, rock etc) or about 100mm of foam board. That will undoubtibly change the roof line. Guessing with thatch you're unlikely to have facias & gutters to worry about but you'll need to look at how that works on the gable ends and where the roof meets any other parts of the builing.
Kingspan, Ecotherm, Celotex etc. all have detailed technical documentation on how to use their products and meet the regs. If they don't have something on their web sites then you should be able to get in touch with them to find out more about their recommendations.
You need to make yourself a mini expert on this as BC and builders are generally not that clued up on how to meet the regulations. Some of my insulation elsewhere was installed so poorly that it has little or no thermal value.