In pitched roof construction, the insulation can either be placed horizontally above the ceiling thereby creating a ‘cold’ roof space or loft, or placed between or above the inclined rafters, thus forming a ‘warm’ roof space below the ceiling. Where the insulation is positioned in the roof structure using both methods, such as a ‘room-in-roof’, this is termed a ‘hybrid’ roof.
Cold roof voids are generally always required to be ventilated; however, where vapour permeable (or Low Resistance, LR) underlays are specified, the levels required may be reduced. In a warm pitched roof, the void beneath the roof covering and underlay may not require to be ventilated, but this is subject to the type of underlay being used - (High Resistance (HR) or Low resistance (LR) – and the requirement to install a fully sealed Air and Vapour Control Layer (AVCL) above the ceiling in conjunction with an air-open roof covering. In a ‘cold’ roof, If the roof covering is not air-open, then both the roof void above the underlay and the loft space below must be ventilated, ideally at both eaves and high level (ridge). The levels of ventilation required for all roof configurations are specified in BS 5250.