The roof joist / truss space can be insulated using a variety of different insulation types including batt (i.e., mineral wool or fibreglass) or blown-in fibrous insulation (i.e., mineral wool or fibreglass).

The insulation should be held back from the interior surface of the roof sheathing to allow for adequate venting. The BCBC requires 2.5" (64mm) of vent space, measured from the interior surface of the sheathing to the top surface of the insulation. Cross strapping (purlins) above the joists provide for ventilation pathways across the top of the insulation in all directions.

See the Illustrated Guide, R30+ Effective Vaulted & Flat Roofs in Residential Construction in British Columbia published by BC Housing.