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This is the best performing of the roofs and is suited to all applications although it is the most expensive one and does not lend itself so well to remedial applications because it often requires a re–roof or the original roof level needs to be raised. Because the insulation is on the outside of the building (like an overcoat) this illuminates the main roof design problem of interstitial condensation because there are no voids for the vapour to condensate in. |
The main use of this form of roof is when the roof space is habited as an original design (mansard style) or when the ceilings are vaulted or of course when design necessitates a flat roof. Only vapour permeable membranes with a vapour resistance of not more than 0.25MNs/g-1 such as Permavent - 0.15MNs/g-1, Permavent-MAX - 0.15MNs/g-1 or Permavent-ECO - 0.11MNs/g-1. Permavent should be laid directly onto the insulation (usually phenolic boards or multi-layered quilt type of insulation). Wet or damp rafters should not be closed in and of course there should be no voids in insulation. Building regulations (L1) make such a requirement for insulation that it resultedthat in rafters being considered as a thermal bridge and require for insulation to span over an exposed edge. Permavent breather membranes should be laid onto the insulation with a counter batten to allow for eaves to ridge ventilation and wind driven moisture runoff.
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