Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 39 impermeable materials are generally damaging, if permeable materials are reintroduced without reinstating the original protection (such as overhangs), their exposure to extensive and persistent wetting will lead to fungal degradation, loss of cohesion and frost damage. Furthermore, heat loss through persistently wet daub, render or brick is much greater. Recent changes in weather patterns may also create greater problems for poorly protected buildings. It is therefore an essential element of any building upgrade (particularly for timber-framed buildings) that adequate overhangs and other protective measures are re-introduced, even where the evidence for them is inconclusive. Another important issue is that moisture content is critical and often finely balanced. Typical ambient moisture content of timber in a well maintained building is around 16 per cent (lower if heated). This tends to rise to around 18–20 per cent in well-maintained external walls. Many fungi will germinate at around 27 per cent, but can survive down to 23–24 per cent. Deathwatch beetle thrive where there is or has been fungal activity and can survive in timber down to 16 per cent moisture content or lower. Controlling water penetration, condensation and evaporation are therefore critically important, and using the wrong materials or details might raise the moisture content by just a few per cent and risk starting or re-starting degradation. Equally, reintroducing the right materials and detailing should lower the moisture content by just a few per cent into the safe zone. SUMMARY 1 The decision whether to retrofit insulation, and if so, which approach to adopt, cannot be taken in isolation. A detailed appraisal of the building, including the historic significance of the timber frame, infill panels and other features, as well as an accurate condition assessment, must be carried out. 2 Most traditional timber-framed buildings will be listed. There should be discussion at an early stage with the local conservation officer about the problems identified and proposed remedies. 3 Upgrading the hygrothermal performance of timber-framed walls by retrofitting insulation is very difficult and can rarely be achieved without significantly compromising the historic significance and/or appearance of the building. Any potential benefits in terms of cost saving, comfort and reduced carbon emissions need to be weighed against the initial cost, loss of historic fabric and potential for further degradation of historic fabric. 4 Where timber-framed walls retain a high proportion of original or historically significant fabric, retrofitting insulation should be considered a last resort and only used when other potential improvements have been explored. 5 Heat loss through the various materials that make up a relatively thin timber-framed wall is often compounded by air leakage around the edges of panels and through joints in the frame. Minimising uncontrolled air movement is critical and will often prove more effective and less damaging. 6 Alternative measures to upgrade the overall performance of the complete building should be considered. These might include reinstatement of roof overhangs and fitting of pentice boards, removal of impermeable materials and finishes, and measures to reduce wind exposure. 7 Timber-framed walls generally have low thermal mass and high uncontrolled air penetration. Heating systems that make use of large internal masonry stacks or stone floors as heat stores are often more effective than systems that heat the air via conventional radiators. Radiators should never be placed against external timber-framed walls. ROBERT DEMAUS BEng MSc (Timber Conservation) specialises in the location, assessment and conservation of structural timber in historic buildings, combining advanced technologies with historic knowledge and practical experience, working for national institutions and private clients throughout Britain and abroad. The severely degraded timber framed end wall has been strengthened and protected behind weatherboarding, with a layer of breathable multi-layer insulation included.