Insulation Above And Between The Rafters – Design Guidance
The underlay should have a vapour resistance of no more than 0.25MNs and can be draped over the counter battens or directly over the insulation. The counter battens should be at least 38 x 50mm and to avoid thermal bridging the roof insulation should be snug against the walls.
If there is a risk of condensation, then a vapour control layer should be used and a eaves carrier might be needed to prevent ponding water. No ventilation beneath the underlay is necessary if there is sufficient air movement that allows moisture to escape.