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.