Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 35 INSULATION IN TIMBER-FRAMED BUILDINGS ROBERT DEMAUS T HERMALLY, TIMBER-FRAMED walls generally perform badly compared with other traditional construction, and struggle to meet modern expectations. This article addresses the problems and risks associated with retrofitting insulation to upgrade their thermal performance. It focusses on cases where the timbers are exposed externally, which are usually the most problematic, but also considers timber frames which are concealed behind cladding (either of the same period or later). There are circumstances in which retrofitting insulation to a timber-framed wall is acceptable and beneficial, but other measures might prove more cost-effective and less damaging. To determine the best way forward, survey and analysis should be carried out by an independent consultant rather than by a materials supplier or contractor. As well as comfort, cost-saving and environmental gain, many other factors must also be considered, including: • The historic significance of the building as a whole, as well as the relative significance of individual elements, and the degree to which retrofitted insulation will alter it • The condition of the building fabric and the nature and extent of any interventions (other than thermal insulation) that might be necessary • The causes of any existing degradation and how these might best be remedied • The current hygrothermal performance of the timber-framed walls and the building as a whole • The ‘landscape value’ of the building and the potential impact of any change to its external appearance • The performance of heating and hot water systems and the cost benefit of upgrading • The condition and efficiency of existing insulation, for example in roof spaces and floors, and the cost benefit of upgrading • The potential for introducing cost-effective and reversible new elements such as secondary glazing that do not involve significant harm to historic fabric • The building’s current use and the occupants’ expectations. The absolute and relative importance of these and other factors will vary greatly, not just between buildings, but between areas of the same building. The thermal performance of a timber-framed wall is not only controlled by its component materials. Condition, orientation and exposure will have a far greater effect on a 100mm thick timber- framed wall than on a 225–350mm brick wall. Moisture retention within the wall is also critical to its thermal performance. The large original verge overhangs of this historic timber-framed house protect the wall below, while an angled ‘pentice’ board above the ground floor window sheds water away from the wall below. (All photos: Robert Demaus)