Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 7 Victorian end-of-terrace in New Bolsover, Derbyshire before (above) and after (below) installation of loft insulation, secondary glazing, internal wall insulation and insulation of the suspended timber ground floor. These thermal improvement measures increased the building’s SAP rating from E (46) to C (73). operate as efficiently as possible (‘Lean’). The final consideration is supplying energy requirements from renewable sources to minimise greenhouse gas emissions (‘Green’). Although this philosophy can be applied in principle to existing buildings, a more nuanced approach is needed and the priorities will differ. For example, while the ‘fabric first’ approach (which focusses on achieving a high performance building envelope) makes perfect sense for a new building, in a historic building this may be neither practicable nor desirable. Instead, effective, cost-efficient and less risky measures that have minimal impact on heritage significance might be identified. Such measures include improving building services and controls, changing the way a building is occupied, used and managed, and questioning current expectations and standards to find out what is really necessary. It is important to remember that success cannot be achieved by technical means alone – building owners, managers and occupiers play a crucial role and should be fully engaged in plans for saving energy at every stage. Where building fabric improvements such as reducing uncontrolled air infiltration or adding insulation are considered desirable and feasible as part of a whole building energy strategy, careful consideration must be given to minimising the risks of unintended consequences. For example, if adequate provision is not made for ventilation, making a building more airtight can result in poor indoor air quality, with consequential health risks for the occupants. And the failure to remove excess moisture generated by activities within the building can lead to condensation and mould. Similarly, the building may be harmed if added insulation adversely affects its benign ‘hygric balance’ (water in = water out) leading to a build-up of moisture within the fabric. Moisture problems caused by poorly designed and badly installed external wall insulation, or pre-existing building defects which allow rain to penetrate and become trapped are already becoming evident in some retrofitted buildings. In some of the worst cases buildings have been rendered uninhabitable. The interactions between a building and the internal and external environments are complex and dynamic. It can be difficult, therefore, to fully predict the effects of particular retrofit measures and to assess the technical risks with any degree of certainty. Although the risk of moisture accumulation can be assessed using numerical models – a range of software applications of varying degrees of sophistication exists for this purpose – there is very little empirical evidence to validate the models. Concerns about the risk of moisture accumulation associated with retrofitted wall insulation have prompted Historic England and others to obtain data from systematic site- and laboratory- based observations conducted over extended periods. The aim is to better understand the hygrothermal behaviour (heat and moisture transfer) of building elements and the effects of energy efficiency retrofit measures. VICTORIAN END-OF-TERRACE, NEW BOLSOVER New Bolsover is a model village built by the Bolsover Collier Company in 1891 on the outskirts of Bolsover, Derbyshire. It comprises 206 two- and three-storey brick houses (Grade II listed) arranged in double terraces on three sides of a square village green. In 2011 Historic England (then English Heritage) leased an end-of-terrace two-storey brick house and carried out a package of measures to improve the energy performance of the building envelope. This included loft insulation, secondary glazing, internal wall insulation and insulation of the suspended timber ground floor. Two types of wall insulation were used for comparison: a non-hygroscopic, vapour- closed system (polyisocyanurate or ‘PIR’), and a hygroscopic, vapour-open system (wood fibre). Hygrothermal monitoring set-up at New Bolsover. Wooden blocks (or dowels) are a convenient method for assessing moisture content in historic building materials. The moisture content of the timber equilibrates to the relative humidity of its surroundings and can be determined (within limits) from measurements of electrical resistance. The humidity of the surroundings can then be deduced.