Slate is a wonderful forgiving roofing.
Insulation slate roof.
If you create an air space under the roof sheathing boards as dana suggests there is no need to have any soffit vents or ridge vent.
Understand headlap read about the common mistakes people make when installing slate roofs.
Where the felt is impervious any contact condensation will run down the felt and make the insulation damp.
Insulation is often laid in contact with the roofing felt.
Read about how to install starter courses take a look at common contractor errors watch some video clips read about drip edges and cant strips if necessary look at some of the nuances that may be pertinent to your installation such as cleats on aprons cleats on.
Ice water shield.
A well ventilated roof space is good this stops your timbers rotting.
It is vapor permeable and air leaky.
The materials used to insulate under the roof include batts of mineral or glass wool insulation held in place by battens of wood attached to and across the rafters alternatively polystyrene.
Insulation laid so that it covers the eaves significantly reduces ventilation to the roof.
The underside of the roof sheathing will dry rapidly to the exterior through the slate due to ventilation drying and diffusion.
Check the depth between your trusses and the slates then install the correct thickness of kingspan in the gaps allowing for a 25mm air gap between the kingspan and the slates.
You would first have to create a vent channel under the slates by installing 1x1 sticks at the top of each rafter bay followed by stiff material like rigid foam or plywood.