The Cold Deck is the most common but also the least thermally efficient design of roof insulation.
In the case of a flat roof the roof deck and waterproof membrane are supported on joists, the internal ceiling is attached to the underside of the joists and should have a vapour barrier to stop condensation penetrating the internal decor. Mineral wool insulation is then installed between the rafters leaving a minimum 50mm (2") air gap above the insulation for air flow.
Good airflow is very important in a cold deck design allowing warm moist air to escape. Condensation can lead to rotting deck and ceiling joists and also cause damp insulation and ceilings. Good external ventilation should be installed in the form of soffit vents.
