The suitability of the remaining plaster at a National Trust Farm, at Levens, to carry softwood fibreboard insulation and suggested insulated wall build-ups for this building

M Womersleys recently provided technical support to a project at a farmhouse at Levens, in the Lake District. The National Trust, and their surveyor and builder, wish to improve the building, by removing inappropriate finishes, carrying out repairs, and insulating the building, to create a more breathable and healthy property. We assessed the suitability of existing wall finishes to accept vapour permeable softwood fibre board insulation materials and lime plasters and suggested the most suitable insulated and breathable build-ups for the walls, for this building in this location.

Old lime plaster can be used as a suitable background to install softwood fibre board insulation, but this can only take place if the plaster is sound enough to carry glued on boards or will not break down if it is drilled through to install insulated fixings. In addition, if the lime plaster has been levelled or over skimmed with modern gypsum plaster this gypsum layer will fail behind the boards. We also made suggestions on how to reduce cold bridging within a spice cupboard and dresser, that were both built into the walls.

Because of the degree of exposure of all external walls except those areas sheltered by the internal environment of an adjacent barn, it has been suggested that the walls should not be over insulated, which may cause decay in timbers embedded in these walls and prevent any drying out of the fabric for months on end. We suggested a compromise of a vapour permeable insulation giving a reasonable insulation levels, with good airtightness, and a u value of on average of 0.55 be used and provided appropriate specifications and diagrams for guidance.