If 180mm is to be installed then 100mm board might be introduced between the rafters there has to be a minimum 25mm air gap between the insulation and the underside of the tiles or slates with an 80mm board running across the rafters.
How to insulate a roof against heat.
With a warm roof the usual process is to split the insulation into two layers.
Another best practice is to have a vented air cavity directly under the roof.
Many homes have at least some access to this space and because of the size attics can hold quite a bit of insulation.
Like a canopy a vented roof basically shades the rest of the house.
Don t forget to insulate the loft hatch.
This usually involves attaching a block of polystyrene insulation to its upper side and ensuring any gaps around the side of it are sealed by applying.