The Building Conservation Directory 2023

138 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 This method may be necessary in some instances. It will allow new insulation to pass behind the back of the steel, allowing for better thermal cohesion. However, this methodology is very heavy-handed and not necessarily appropriate. If embedded timbers, beams and joist ends can be left in existing walls, then treatment with a boron paste should be considered. While there is some opposition to using chemicals in historic buildings, boron paste can help preserve embedded timber and support its retention. Gaps around the timbers should be filled with a lime-based mortar to prevent humidity from entering the pocket that contains the end. Vapour permeable thin internal wall insulation may also help reduce moisture risks or thermal bridging at the joist ends. DESIGN There is no universal approach to retrofitting insulation into a traditional building. For example, evaluating a group of Georgian buildings in London will use a different methodology to a coastal cottage in the west of Ireland. Therefore an appropriate risk assessment must be carried out prior to any work. Dynamic hygrothermal analysis such as WUFI can help assess moisture-open constructions where wind-driven rain or other moisture sources are present. Hypothetical or theoretical calculations are often used to understand and evaluate the movement of energy and moisture through solid walls. However, consider that these may differ significantly from the actual in situ values. Similarly, the value given to an in situ material such as brick can vary from the material used in service. Nevertheless, this approach can allow for a greater understanding of the risks. Contemporary building practices include vapour barriers and impermeable insulations, which tend to trap moisture. There is a long-standing approach of installing a vapour-closed insulation system with a cavity to allow ‘breathability’, often supplemented with a vapour barrier. However, this ‘dry lining’ detail is risky and likely to cause interstitial condensation, particularly where the building is exposed to wind-driven rain. Traditional buildings are best suited to a moisture-open approach, which means both vapour permeable and capillary active. Less is more when it comes to insulation. While uniformity or thermal cohesion is essential. Increasing insulation may be counterproductive when it comes to thermal bridging spots. A maximum thickness of 60mm or approximately 0.6–0.7 W/m 2 K is generally considered sufficient on a traditional building. This approach may allow thermal transfer into the masonry and enable drying potential, particularly to the embedded timber ends. Permeability is also a factor that must be considered. The increased use of non-permeable insulation is becoming an issue resulting in fungal outbreaks in traditional buildings. We might consider that foil-backed PIR has a far greater R-Value than wood fibre insulation. This means that high vapour diffusion resistance, such as PIR, is likely to cause a build-up of moisture within the walls in the long term. Therefore, the hygroscopic and vapour permeability of the chosen material will significantly impact the building fabric and embedded timber structure. GOING FORWARD While there are gaps in knowledge and understanding of how insulating traditionally built structures perform in the longer term. The results of some monitoring programmes allow a better understanding of the performance of the building envelope. Hygrothermal modelling of Shrewsbury Flax Mill Maltings and information on a Victorian end-of-terrace in New Bolsover provide beneficial information. There is also ongoing monitoring of infill panels in timber-frame construction, which is significant research for timber-frame construction. The newly published ‘Investigation of Moisture and its Effects in Traditional Buildings’ is also a step in the right direction. As we adapt our historic buildings for a sustainable future, making informed decisions by holistically understanding how buildings perform is fundamental to their preservation. Recommended Reading Bristol City Council, A Bristolian’s guide to Solid Wall Insulation , bc-url.com/ bristol, 2016 Historic Environment Scotland, Technical Paper 15; Assessing risks in insulation retrofits using hygrothermal software tools: Heat and moisture transport in internally insulated stone walls , bc-url. com/historic-environment-scotland- technicalpaper-15, 2015 English Heritage, ‘ Building Environment ’ , Practical Building Conservation Series , bc-url.com/english-heritage-building- environment, Ashgate Publishing, Farnham, 2014 Ryan, Carole, Traditional Construction for a Sustainable Future , Routledge, 2011 Baker, Paul, Shrewsbury, Shropshire: Hygrothermal Modelling of Shrewsbury Flax Mill Maltings , bc-url.com/ historicengland-hygrothermal- shrewsbury, 2016 Whitman, Chris J et al, Hygrothermal monitoring of replacement infill panels for historic timber-frame buildings: initial findings , UCL Open, bc-url.com/ hygrothermal-monitoring-timber-frames, 2022 Historic England, Investigation of Moisture and its Effects in Traditional Buildings , the Royal Institution of Chartered Surveyors and the Property Care Association, 2022 PETER BARRETT is the director of PJ Barrett & Co, damp and timber decay specialists based in Ireland and has 20 years of experience in the building industry. He studied building surveying with a specific emphasis on dampness, timber decay and its remediation and holds an MSc in Building Conservation from the University of York. A building that was insulated with inappropriate internal wall insulation and, as a result, allowed conditions for wet rot fungus to flourish on all embedded timbers The benefit of drone technology when reviewing a building