The roofspace can often present the “low hanging fruit” in terms of energy upgrade potential in a historic building by comparison with other heat loss elements such as walls, windows and floors. Unless the roof presents itself as a cold roof with a large accessible attic working space and a flat ceiling which can easily accommodate insulation layers, historic roofs can often present a range of difficult-to-insulation junctions and bellcast sections of sloping roof.
While some guidance indicates the possibility of inserting rigid insulation board into bellcast sections of roof from above, this can be made problematic where the ceiling collars tie in to the rafters – the space available at this junction is narrower than the space between each pair of rafters. A semi-rigid insulation batt product can be squeezed into the space past the collar tie, however it can be very difficult to ensure that the insulation is kept tight to the edge of the rafters and an appropriate space between the insulation and the underside of the roof finish is maintained.
A strategic decision in this instance has typically to be made particularly in the context of a larger programme of works as to whether roof or ceiling finishes can be removed. If re-roofing works are proposed as part of the project, the insulation works can be undertaken from above. If non-original plasterboard ceilings are present below roof level, consideration could be given to removal of these ceilings and insulating from below. In these situations, these difficult areas of roof can be more effectively insulated. Lath and plaster ceilings are part of the historic fabric of buildings and should be generally be retained.
The provision of roof ventilation can often be a more challenging issue than the installation of the insulation material itself. Roof ventilation products such as ventilators in roof slopes and dry-fix ridge systems can detract from character. The provision of eaves ventilation products to a traditional closed eaves can be problematic in terms of the available space below the bottom slate and the fixing of the ventilators.
The images below illustrate this point where there was a desire to reinstate a traditional closed eaves detail at a building that was both a protected structure and a recorded monument. The use of proprietary eaves ventilation products, the incorporation of battens and the related alteration of the wall tops was considered inappropriate. It is noted that many “breathable” roof underlays require secondary roof ventilation as part of their certification. In this instance a roof underlay that did not require secondary ventilation and ancillary ventilation products was selected. The slates were fully bedded in mortar at eaves level to reinstate a traditional detail and with the roof underlay cut around the top batten and drained out between two courses of slate above wall tope level.
These Historic England documents below are a particularly useful resource in relation to insulation approaches with clear diagrams and details: