Insulation of a flat joist-type roof ceiling above the sheathing

Another approach is to place the insulation above the sheathing, as is commonly done with low-slope roofs (Figure 27). Insulation exposed to the weather must be impermeable to moisture. Polystyrene insulation is commonly used on the outside of roof sheathing and protected by the roofing membrane

Figure 27.

Source : Canada Mortgage and Housing Corporation (CMHC)