Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 37 of limewash applied. The entire fabric was therefore very breathable, allowing any moisture that entered to readily evaporate, and moisture levels in the wall generally remained below the point at which the various materials would degrade. The walls were protected by large roof overhangs and pentice boards (see title illustration), but over the following centuries, these were lost, causing the walls to be wetter more often and for longer periods. As a consequence, the wattle and daub began to degrade rapidly and the timber more slowly. In the 18th and 19th centuries timber-frames were often concealed behind facades of weatherboard, brick, tile or lime render. Early renders were lime-based and breathable: later renders were often much less breathable, such as Parker’s Roman cement which was patented in 1796. Where frames remained exposed into the 19th century, the degraded wattle and daub was often replaced with brick, which tended to exacerbate degradation. Increasing use of cementitious renders, impermeable paints, damp-proof membranes and mastic sealants in the 20th century tended to reduce breathability and trap water, increasing degradation and heat loss. More recently, economic and environmental pressures to improve thermal performance have become increasingly important, but often poor detailing and inappropriate materials have exacerbated decay. In the 21st century there has been a growing understanding of the need for buildings to breathe and a consequent move to more permeable materials. The crucial point is that impermeable modern finishes and sealants not only cause significant and continuing damage to the timber frame and other historic fabric, they also greatly diminish the thermal performance of the wall. The condition of the wall and its hygrothermal behaviour are intimately linked. Unless faults are remedied, the introduction of insulation may be of relatively little benefit and can greatly increase the risk of further deterioration. Only when the detailed survey has been completed can the advisability of retrofitting insulation be evaluated and the best method selected. There are essentially three options for retrofitting insulation to an exposed timber-framed wall; externally, internally or within the depth of the frame. WITHIN THE FRAME Given that timber-framed walls are often less than 100mm thick, insulating within the depth of the frame almost inevitably involves loss of the existing infill material. Original wattle and daub should be retained and repaired if possible, but where there is a later brick infill, its historic and aesthetic significance and its condition may affect the decision. Where there is evidence of significant degradation, a good case can be made for its replacement with a more sympathetic and better performing material. Where the timber frame requires repair that involves removal of the infill, there is an opportunity to introduce more sympathetic and better performing infill. It is now generally accepted that infill panels should be breathable and vapour permeable throughout their thickness, but there are many theories about the best materials and techniques. Many recommended systems involve complex combinations of materials including synthetic edge seals, breather membranes and vapour barriers, stainless steel mesh, wood-wool substrates and softwood sub-frames. Systems such as these may work better in theory than in the variable conditions found on site, where quality control may be difficult, particularly when the timber frame is neither straight nor in perfect condition. As a rule, the simpler the method and materials, the more likely they are to function predictably and reliably. There is great merit in using methods and materials as close to the original wattle Historically, many timber-framed buildings were rendered to improve their weather-tightness. Some of the visible render is lime-based and probably early 19th century, other sections have been replaced with a cementitious render in the 2oth century.