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.

Figure 26

Figure 27

Figure 28