Thanks to the efforts of the owner in the 1980s, the roof (constructed of slate tiles and sarking boards over pine rafters) was in remarkably good condition, albeit entirely uninsulated. After considerable modelling in collaboration with an architectural technologist, we chose to insulate between the rafters with solid insulation, leaving a ventilated air gap under the sarking, then added tri-foil insulation above the rafters before sheeting the whole structure. With the installation of a new ventilated roof ridge and concealed ventilators for the air gap, this gave us a delivered u-value of about 0.18W/m2k, significantly ahead of Scottish targets for retrofit of 0.35W/m2k and close to the latest standards for new build.
The roof space had been a bat roost for two species of Pipistrelle and Brown Long-Eared Bats and we asked the Bat Conservation Trust (BCT) to help us both identify the species involved and what we could do to continue to encourage their presence in future. Having looked at bat tiles and boxes, we eventually chose to set aside one end of the attic as an undisturbed roost area (now inevitably known as the bat cave). This area is wired for IR cameras should we wish to monitor activity.