Heritage Retrofit

4 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION CONSERVATION PRINCIPLES AND HERITAGE IMPACT METHODOLOGY ‘Significance’ is the collective term used by heritage professionals to encapsulate the diverse heritage values that can be ascribed to a building. These values may include artistic, symbolic, historical, social, economic, scientific and technological attributes. Assessment is the key to articulating heritage values, whether for statutory designations or local recognition, and the preparation of statements of significance is generally a pre-requisite where interventions are proposed. Statements of significance aim to: • identify the ‘character defining elements’ of a building and its curtilage • describe the degrees of significance (typically, from high to none) that attach to setting, form and appearance, components (such as doors and windows) and material fabric • facilitate accurate and transparent assessment of the overall and detailed impact of retrofit measures (from none to high). A successful retrofit procedure additionally requires: • detailed examination of existing needs for repair and conservation • assessment of the thermal characteristics and performance of the building envelope, for which expertise in thermal imaging is a crucial advance on theoretical U-values • analysis of heating and electrical systems • understanding of moisture effects and humidity • assured competences in the requisite disciplines. A main objective of the 2012–16 European research project Energy Efficiency for EU Historic Districts’ Sustainability (EFFESUS), was to develop a heritage impact assessment methodology for the selection and prioritisation of cost- effective life cycle energy efficiency improvements at the urban district scale. For this, given that listed buildings account for less than three per cent of the total, an inclusive definition of historic urban district was adopted. The definition covers almost a quarter of the building stock: ‘a significant grouping of old buildings, built before 1945 and representative of the period of their construction or history, and comprising buildings which are not necessarily protected by heritage legislation’. An essential premise was that building-scale approaches are inefficient and costly, and urban strategies which identify representative typologies of buildings will support the economies of scale associated with mainstreaming retrofit technologies, as well as achieve the reductions in greenhouse gas emissions anticipated in EU directives (targets which the UK is independently committed to achieving under the Climate Change Act 2008 ). Adopting the principle of holistic understanding of the suitability of specific retrofit measures in any given situation, heritage impact assessment is one of six complementary modules in the EFFESUS project. The other modules are operational energy, indoor environment (air quality and humidity), fabric compatibility, embodied energy (of retrofit measures) and economy. A complementary component of the EFFESUS project has been the research and testing of fabric retrofits incorporating new technologies. These include high-performance insulating lime mortars for use externally as render and internally as plaster, silica aerogel fibre insulation as infill to cavities behind internal dry linings, and advanced window systems with integrated air supply valves and shading blinds connected to building management systems. Essential to the overarching context is that heritage values are maintained and interventions and their consequences have minimal ecological impact. Minimum intervention is a core conservation principle and encapsulates both objectives. It is also essential that all actors are focussed on elaborating properly considered retrofit solutions that are long- rather than short-term and do not respond to individual concerns (such as simply reducing heating bills) while provoking others (such as moisture and health issues). Retrofit solutions should be communicated to building owners and occupants in ways that express what they can do rather than what they can’t and, most importantly, why they have been chosen. SAMPLE RETROFIT MEASURES Retrofit measures fall into two main categories: those that improve the thermal and energy performance of buildings (fabric and services) and those that change the energy supply source from fossil fuels to renewables, whether at the individual building scale or at the urban district level. If the reduction of carbon emissions is the sole or primary objective, then a balanced approach that converts the energy source to renewables can limit the need for fabric interventions, especially those which would impact prejudicially on a building’s heritage significance. In this, rapid advances are being made across the multiple options for renewables – solar, wind, biomass, ground, air and water source heat exchangers, micro hydro- electric and others. Advances are also being made in defining methodologies for cradle-to-grave carbon emission audits to confirm whether or not certain technologies really are ‘green’ as opposed to just appearing to be so. For fabric retrofits, the starting point is to fully understand what are the weakest areas and components of buildings (which are not necessarily those promoted in government programmes such as cavity-wall insulation and window replacement), and what are the most cost- effective and sustainable ways of dealing with them: from roofs to walls and floors, windows and doors, and involuntary leakage. New solutions such as those researched for the EFFESUS project need to be promoted in tandem with the recovery of traditional solutions. The weakest links in buildings are often their roofs – frequently simple to remedy with ecologically-friendly insulation materials such as sheep’s wool – and windows. From a heritage significance as well as an ecological perspective, however, windows can be the most challenging to retrofit. What merit is there in replacing a repairable 250-year-old timber window with one which may, in whole or part, only last Lörrach, Baden-Württemberg, south-west Germany: biomass-fuelled urban-district heating plant. District energy systems, whether for heating/cooling or electricity, are more cost-effective and sustainable and avoid impacting on the heritage significance of individual buildings.