Various types of rigid insulation can potentially be used above the sheathing in this assembly, including rigid mineral fibre, extruded polystyrene (XPS), expanded polystyrene (EPS), and polyisocyanurate (polyiso).

Insulation placed at the interior can be batt (i.e., mineral wool or fibreglass), or blown-in fibrous insulation (i.e., mineral wool or fibreglass). The ratio of interior-to-exterior insulation R-value in this assembly must be carefully considered to avoid risking condensation at the underside of the roof sheathing.

Split-insulated roofs should be designed using hygrothermal design software or calculations that consider the climate zone and interior conditions that the assembly will be exposed to. As a starting point, the assembly should have at least 65% of the total nominal insulation R-value placed at the exterior of the roof sheathing.

Interior vapour control is also important in this assembly in order to prevent excessive ambient moisture from reaching the sheathing, without trapping moisture that may enter the assembly.

A relatively more permeable interior vapour barrier such as a smart vapour retarder or vapour retarder paint could be used. Depending on the insulation ratio and results of hygrothermal calculations, the assembly may not need interior vapour control.