I have a bathroom above an integral garage, with various pipes and some cabling above the latter's ceiling. There's absolutely no insulation in the void and I'll probably be installing Celotex or polystyrene and then plasterboard. (Some of this needs to be replaced anyway following a leak.)
I think that I've got it all sussed out but am puzzled by one admonition to seal all joins and gaps and another to ensure that "ventilation must be provided above the insulated ceiling directly through the cold void".
I'm familiar with the need for ventilation in an attic but don't know how to ensure it in this case, the void being topped by floorboards and compartmentalised by joists.