Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 9 data from the interface between the insulation and the wall. During the winter months the room is heated and humidified to simulate occupation. Internal air temperature and relative humidity are monitored while a weather station records meteorological data. In addition, there are gauges to measure the amounts of wind-driven rain striking the elevations of the building. A little over a year’s worth of data has been gathered and is currently being analysed. However, differences in the hygrothermal behaviour of insulated and uninsulated walls, and variations resulting from the differing orientations of the walls are already apparent. Interestingly, the largest differences between summer and winter temperature and humidity levels were observed in the exterior sections of the insulated walls. In contrast, the scale of the fluctuations in the exterior sections of the uninsulated walls was generally smaller. Further investigation will be needed to determine whether the magnitude of these fluctuations would be sufficient to increase the vulnerability of fragile brick surfaces to deterioration over time. WUFI Measured data obtained from New Bolsover and Shrewsbury Flaxmill Maltings is also being used to investigate factors affecting the accuracy of heat and moisture transfer simulations using WUFI software ( Wärme Und Feuchte Instationär or ‘heat and moisture transiency’) in a project being carried out on behalf of Historic England by Dr Paul Baker at Glasgow Caledonian University. The ability to predict the hygrothermal behaviour of building components is important in assessing and managing moisture risks. WUFI software has been developed by the Fraunhofer Institute for Building Physics, Germany. It complies with BS EN 15026:2007 which sets out minimum requirements for simulation software use to predict one-dimensional transient heat and moisture transfer in multi-layer building components exposed on both sides to transient climate conditions. In the first phase of the project, WUFI Pro 5 software was used to simulate the hygrothermal behaviour of brick walls, both insulated and uninsulated, at Shrewsbury Flaxmill Maltings over a period of 30 years (1960–1990) using historical meteorological data recorded at a nearby weather station. The simulations were carried out using the material properties of two bricks from the WUFI database (‘hand-formed brick’ and ‘historical brick’) plus the measured material properties of a third brick from the main mill building. Four types of insulation system were modelled: • wood fibre • mineral wool without an air and vapour control layer (AVCL) • mineral wool with AVCL • PIR. Results of the various simulations were then compared. There were significant differences between the results obtained when the measured properties of the Flaxmill brick were used instead of the WUFI database values. Clearly, it is better to use the measured properties of traditional building materials for hygrothermal simulations – there will always be uncertainties when using alternatives from the WUFI database. (This observation has also been made by other researchers.) It is a drawback of the application that the database contains no traditional UK building materials. After comparing the simulation results obtained for the different types of insulation, wood fibre – which is hygroscopic and has some vapour diffusion resistance – appeared to be the best of the four systems. Mineral wool, although it has very low vapour resistance, is non-hygroscopic and therefore unable to buffer moisture. The insulation systems with higher vapour diffusion resistance – mineral wool with AVCL and PIR – appeared to cause moisture to accumulate within the walls in the long term. Altering the rain adherence factor in the model had a significant effect on simulation results. The actual rain adherence factor at the Flaxmill is unknown, and may vary depending on the intensity of wind-driven rain. It may be possible to ‘calibrate’ the model by adjusting the rain adherence factor based on the site measurements of wind- driven rain. These, and other unknown boundary conditions led to a high level of uncertainty about the simulation results. The next stages of the WUFI project will include sensitivity analysis of input parameters and the modelling of the walls at New Bolsover so that a direct comparison can be made between the simulation results and the measured data gathered over the past five years. In due course it will also be possible to compare the simulation results with measured data from Shrewsbury Flaxmill Maltings to provide further validation of the model. (A research report on the first phase of the hygrothermal modelling project can be downloaded from Historic England’s website, see Further Information.) LOOKING AHEAD In addition to the research described above, Historic England’s Building Conservation and Research Team is also investigating the effects of added insulation on the hygrothermal behaviour of roofs and suspended timber ground floors, including the role of ventilation in maintaining moisture at safe levels. Building physics is complicated. Dr Paul Baker observed recently ‘It’s not rocket science – it’s harder!’ And there are still many gaps in our knowledge and understanding. Therefore, a very welcome and timely development has been the launch earlier this year of the UK Centre for Moisture in Buildings (UKCMB). This not-for-profit organisation will work with partners from academia, government, industry and the public to substantially improve the way moisture risk is understood and managed in the UK. Watch this space. Further Information Historic England, A Retrofit of a Victorian Terrace House in New Bolsover: A Whole House Thermal Performance Assessment , 2015 (http://bc-url.com/whole-house ) Historic England, Ditherington Flax Mill: Hygrothermal Modelling , 2015 (http://bc-url. com/ditherington) Historic England Research Report, External Wall Insulation in Traditional Buildings: Case studies of three large-scale project in the North of England , 2014 (http://bc-url.com/ewi ) Historic England technical guides on energy efficiency and historic buildings are available at https://historicengland.org.uk/advice/ technical-advice/energy-efficiency-and- historic-buildings Historic Environment Scotland guidance on saving energy in traditional buildings is available at https://www.historicenvironment. scot/advice-and-support/your-property/ saving-energy-in-traditional-buildings/saving- energy-guidance Sustainable Traditional Buildings Alliance, A Bristolian’s Guide to Solid Wall Insulation , BCC, 2015 (http://bc-url.com/bristol ) Sustainable Traditional Buildings Alliance, Planning for Responsible Retrofit of Traditional Buildings , 2015 (http://bc-url.com/retrofit ) UK Centre for Moisture in Buildings ww.ukcmb.org IAIN McCAIG is senior architectural conservator at Historic England. He studied architecture before specialising in building conservation and has many years of experience in both statutory conservation bodies and private practice.