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.
Vapour control layer (VCL)
All non vented roofs must incorporate a VCL such as high performance Permavent Reflect. A VCL is usually positioned just behind the plaster board. BS:5250 recommends that a VCL should to be a minimum 500 gauge polythene (plastic DPM) sheeting and all corners and laps should be minimum 75mm and taped with 'Tactape'. With a warm roof it is acceptable to tape the joins of a foiled insulation with the appropriate foiled tape ensuring all joins and peripheral edges are sealed. Vapour will travel through or manifest itself in the joins of foil backed plasterboard if not detailed correctly. Given the difficult nature of vapour sealing foil backed plasterboard we do not recommend its exclusive use as a VCL. Care should be taken with cables and pipes that break the VCL.
Air sealed roofs
If the dwelling is to be air tested then all laps and joins should be carefully taped and peripheral edges sealed. All pipes and apertures should be sealed and an air tight loft hatch used.
Air permeable roof coverings.
As vapour travels through Permavent it must then be released to the atmosphere. Most roof coverings (concrete, plain tiles etc) allow what is termed 'fortuitous ventilation' and are classed as air permeable roof coverings. This means that air can travel around them and vapour will not get trapped in the batten space between the breather membrane and tiles causing 'interstitial condensation'. Some metal sheet and man made slate roof coverings may not allow sufficient air circulation and the batten space should therefore incorporate additional ventilation such as counter battens with ridge and eaves ventilation.