Insulation between the rafters can be designed in two ways:
• ‘Breathing’ roof with Low Resistance (LR) underlay
insulation fully fills the rafter space without an airspace between the insulation and type LR underlay. Counterbattens are needed to ensure wind driven rain drains freely to the gutter.
Insulation over the rafters provides a completely insulated external envelope to the roof construction, and allows the maximum usable space within the roof enclosure.
insulation boards are laid across the rafters.
Counterbattens are fixed through the insulation boards and LR underlay into the rafters.
Further insulation is placed between the rafters to achieve the desired thermal performance
• Ventilated design with High Resistance (HR) underlay
With this design, a 50mm ventilated airspace is required between the top of the insulation and the tiling underlay. Should the rafter depth be insufficient to accommodate both the required thickness of insulation and the 50mm ventilated airspace, a Knauf PIR Laminate board can be used to underline the rafters and provide additional thermal resistance. This has the added benefit of minimising thermal bridging through the rafters.