Gdw68

Hello All, I’m currently building a 12m x 12m garage/shed with a dividing wall, using 8x2 joists on the roof, and was planning to use 18mm OSB, membrane, then tin, is there any way I can fill the joists full with insulation from underneath? I’m using box section every 4.8m so would struggle to get the joists vented. Would I get away with it? I want to insulate all walls and roof then plasterboard it? Trying to keep it as simple as possible without causing future problems? TIA