The Building Conservation Directory 2023

106 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S TIMBER WINDOWS Conservation and thermal improvement JONATHAN TAYLOR T HE UK is committed to reducing carbon emissions to ‘net zero’ by 2050. According to the UK Green Building Council the built environment is currently responsible for a quarter of all UK greenhouse gas emissions (buildings and infrastructure, excluding transportation), and almost half of these emissions is estimated to come from the heating of homes. New buildings are much more energy efficient than older ones, but by 2050 only around 20 percent of our houses will have been built to modern building regulations. Retrofitting our existing buildings to reduce their energy use is therefore essential if we are to meet the government’s net zero target. Historic and listed buildings will inevitably be part of this solution. However, the effect of some ‘improvements’ can be extremely damaging to historic and traditionally constructed buildings. Double-glazing in particular often involves the loss of the original windows and details, and replacement windows are often both highly damaging to the significance of a building and of dubious value from an energy conservation perspective. Other changes such as solid wall insulation and even roof insulation need to be treated cautiously too, as all too often the results have led to condensation on the elements of structure that are no longer warmed by the heating system, leading to damp, mould growth and decay. Where a building is listed, almost all alterations require listed building consent from the local authority, including insulation and double glazing. In conservation areas, strict controls may also be imposed by local authorities on specific alterations affecting the external appearance of houses through article 4 directions. STATUTORY REQUIREMENTS FOR ENERGY CONSERVATION Energy performance certificates (EPCs) are needed whenever a property is built, sold or rented. The EPC indicates the energy efficiency of the property as estimated by the standard assessment procedure (SAP). The higher the score the lower the running costs are likely to be, with 100 representing zero energy cost. EPC band A (most efficient) corresponds to a SAP score of 92–100, while band G (least efficient) corresponds to a score of less than 20. Listed buildings are exempt, but owners may feel obliged to include their EPC assessments when they sell or lease their buildings. It is generally accepted that this system is heavily flawed because it is geared to reducing running costs, not carbon. Furthermore, the SAP score is based on modern construction technology, and does not reflect the actual performance of traditional and historic buildings. New development such as an extension is also regulated under the Building Regulations. Part L ‘The Conservation of Fuel and Power’ (or Part J of the Scottish Building Regulations) applies strict limitations on the heat loss permissible from new buildings, from extensions, and from existing buildings where a thermal element (wall, floor or roof) is altered or replaced. In an existing building it is usually only the components being altered which are affected by the regulations. However, where a building is being converted to flats or from non- residential to residential and for certain other changes of use, they can apply to the whole building. Part L (or J) does not apply to buildings and extensions which are not habitable. A conservatory for example, may not need to comply if it is separated from the interior of the building by doors. The insulation standards, which are measured in units of thermal transmittance or ‘U-value’, favour double rather than single glazing, but there is a considerable degree of flexibility, and Part L specifically states that double glazing ‘could be inappropriate in conservation work’. Acceptable standards are given by the ratio of window opening to floor area for single as well as double glazing, and variation from standards are also acceptable ‘if compensatory provisions are made’; increases in insulation to the roof space for example, could be used to off-set the heat lost through single glazing; solar gain through south-facing windows may also be taken into account. It is interesting to note that aluminium and PVCu do not perform as well as timber in terms of their thermal efficiency. In addition, both involve a much greater consumption of energy in their production than is used to produce timber – over 45 times as much in the case of aluminium (TRADA figures). Wood from managed plantations, on the other hand, is carbon positive, in that carbon is removed from the air as the timber grows. Detail showing the draught-stripping used at Clovelly Secondary glazing applied directly to the opening light at Clovelly Upgrading the windows at Clovelly, Devon with straight-stripping and secondary glazing by The CosyHome Company