Heritage Retrofit

36 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION be found in brick or stone walls, with correspondingly complex, variable and unpredictable physical properties and interactions. Moreover, the original wide palette of materials is often further complicated by subsequent alterations and additions, such as changes from wattle and daub to brick infill, from lime to cement render and from permeable to impermeable finishes. Many of these variations can occur within a single elevation. By comparison, brick and stone walls are relatively homogeneous and predictable. As a result, desktop heat-loss calculations using standard formulae and computer modelling are less reliable where walls are timber-framed. On-site detailed physical investigation is required. Sensitive infrared thermographic cameras may be used to locate concealed timbers, identify the make-up of infill panels and assess heat loss and damp penetration. Decay-detecting micro-drills are also very useful for these investigations. For such assessments to be of value, the assessor must have a good working knowledge of how the building was constructed, what changes might have occurred since, and the causes and extent of any degradation. Typically, timber-framed buildings were built using freshly felled timber that shrank and moved significantly, particularly over the first 30 years. The timbers were usually left exposed externally and internally and the spaces between the frame were filled with clay- based daub, often finished with limewash. Gaps that formed between the frame and infill as the materials settled and shrank were regularly filled and additional coats This timber-framed house has retained its original eaves and verges, both beautiful and practical. The house must always have been tiled. The depth of thatch shelters the wall below. If the thatch were replaced with tile or slate (as so much was) the wall becomes very vulnerable. These rafters have been restored to their original overhang, greatly improving protection of the wall below. Timber frames are full of joints and cracks through which air (and water) can penetrate. The most effective improvement that can be made to the overall hygrothermal performance is to fill these gaps. A sensitive thermographic camera is the best way of locating them, provided there is a reasonable temperature difference (5–10 o C) between the inside and outside, and preferably when the wind is blowing. It is as important to survey the outside of the building to identify where heat is escaping, as it is internally to identify where cold air is entering. The survey should be repeated when the remedial work has been completed, but is almost meaningless unless carried out in the same weather conditions. Most timber-framed buildings are too air-porous for standard air pressure tests to be meaningful. Optimum methods and materials for gap filling will vary depending on the size and location of the gaps, but should always be flexible and breathable: sheep’s wool pushed into the gap with a thin blade and finished with haired lime plaster can be very effective. Proprietary sealants, mastics and cementitious mortars should not be used. There is a much greater variety of constructional materials and details in timber-framed walls than might