A concrete roof, usually with a suspended ceiling, internally insulated with a foil faced rigid thermoset PIR insulation. The PIR insulation is mechanically fixed directly to the underside of the slab and must be taped and sealed to form a continuous air and vapour barrier preventing internal moisture vapour from condensing on the underside of slabs at below dew point temperature. This solution is often used as a compromise where external insulation is not an available option. Examples would be to the underside of terraces where the insulation should also extend a further minimum 600mm inside the perimeter of the conditioned space above the slab.