Hello, Have recently had two "cold" flat roofs installed: one over a downstairs room and the other over part of the kitchen. The old roof beams were so badly rotted that all were replaced. So no ceilings at present.. I need to get insulation installed and then, presumably, plaster boards fitted. As per the roofers advice' I would like to use 100mm of rockwool insulation between joists; leaving a 50mm air gap below the decking. I see one can buy rockwool of this thickness in "slabs". A couple of question: 1.) how would such rockwool slabs be fixed between the joists? 2,) The joists have holes at the top for the electric cabling: Would these be suffient for ventilation? Thanks in advance, Dave.