A local builder has installed insulation into the sloping ceilings of the top floor of our house. The old ceilings have been removed, and Recticel Eurothane installed between the rafters to within 25mm or less (at the edges, less where underlay sags) of the roofing felt. There is then insulation backed plasterboard fixed below the rafters and skimmed. The roofing felt is described by our builder as semi-permeable; it would have been installed in the late 1980s when the roof was last renewed. There seems to be ample ventilation in the open space above the flat portions of ceilings (which have a vapour barrier underneath wool based insulation), but it is not clear that there is much air flow from the eaves, and because of gables some parts of the roof do not go down to the eaves, but finish in valleys. We are concerned because on cold mornings (especially when frosty and especially on the north side of the roof) substantial condensation is forming on the underlay that is close to the new insulation. This persists for days, and sometimes is sufficient to run down and appear in the ceiling plaster, visible in the room below or running down lighting cables. We are concerned that this will lead to rot. Can the situation be alleviated by improving the access of air at the eaves or possibly through ventilating roof tiles (any substantial work of this kind is likely to involve scaffolding)? Or is the only real solution to take the roof off and install breathable underlay? Or is there anything else that can or should be done?
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