128 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS un-rendered walls, which in turn dried faster than the wall with NHL render. It is almost certainly due to the reduced ability for moisture to dissipate through NHL mortars, although more field work would be needed to confirm this result. Dye penetration tests followed by dismantling indicated that water taken into the wall was not absorbed homogeneously but rather was concentrated at the interfaces between block and mortar. Poor bonding here could lead to formation of capillaries able to absorb WDR, particularly where the blocks are of low permeability. However, it was found that this failure did not manifest for a considerable time. Unsurprisingly, this is more typical of mortars made with NHL binders. CONCLUSIONS This work highlighted the importance of regular maintenance and proactive repair strategies for historic masonry structures. It is clear that the historic removal of traditional renders from solid masonry structures will have increased their vulnerability to driving rain and that, over time (compounded by periods of inadequate maintenance or poor repair decisions), the subsequent rain penetration will have created voids and other failures that will have exacerbated rain uptake still further. Once a wall core has become voided, it is much more susceptible to rainwater penetration and thus free water is more likely to appear on the interior. Although the optimum treatment for any particular building must depend on a number of factors including aesthetics, the Damp Towers research has greatly increased our understanding of the factors influencing long-term behaviour of different types of repair. These factors should be a critical consideration when developing programmes of conservation. Laboratory testing strongly indicated that rendering with a lime-based mortar is the most effective way of lowering the rate of water absorption. This accords well with the field results at Holy Trinity, Challacombe, where rendering the tower produced a dramatic reduction in damp and an equally dramatic improvement in the interior conditions and usability of the building. It also corresponds with the results of recent research by Tim Meek at the University of Stirling, which sheds light on the role of external/internal pressure difference in water penetration in bare stone buildings and the efficacy of lime render in moderating this. The research also showed that there appears to be a case for internal lime plastering. For buildings where rendering is considered unacceptable, and the masonry is to be repointed, it is critical to consider compatibility between the masonry units and the repointing mortar. While grouting may deliver some improvement to voided walls, it is a very complex, expensive and difficult intervention which, if badly carried out, may exacerbate rather than reduce WDR problems, and could be risky in walls where the bedding and core mortars are made of subsoil. Recent evaluation of a number of churches that have been grouted have indicated that some still have ongoing problems of water penetration. NEXT STEPS The mortar mixes used in the laboratory testing at Sheffield Hallam were informed by standard practice at the time, when NHL mortars were widely considered to be the optimum material for use in exposed conditions. This was before research into the properties of NHL mortars was carried out by Christiano Figueiredo at the University of Bath, and before work by David Wiggins at Glasgow Caledonian University which demonstrated the benefits of lime-rich non-hydraulic lime mortars in moisture transport and dissipation. Work by Nigel Copsey and others has also demonstrated that many traditional surface coatings contained very high proportions of lime binder. Current testing at Sheffield Hallam on solid brick walls bonded and rendered with hot-mixed non-hydraulic lime mortar has demonstrated its excellent resistance to floodwater penetration. For buildings suffering from rain penetration, it seems likely that even greater benefits than those seen in the Damp Towers laboratory research could be delivered using lime-rich traditional renders and plasters. It also seems likely that such traditional limebased finishes could improve the thermal performance of solid masonry walls, but to date there has been little research to quantify the benefits. A new phase of the Damp Towers research by Historic England will therefore focus on the impact of traditional lime-rich surface finishes to help create drier, warmer walls. The authors of this paper have summarised the work of many people and organisations over the seven years of the project and would particularly like to acknowledge English Heritage (the research was led by Chris Wood, then Head of Building Conservation and Research Team), Steve Hetherington of Sheffield Hallam University, the University of Oxford, Colin Burns, and Ridout Associates. LIZ LAYCOCK is Professor of Stone Conservation at Sheffield Hallam University. Her research interests are in assessment of the properties and performance of geomaterials. MARIA-ELENA CALDERÓN is Principal Building Conservation Advisor at Historic England, Chair of the IHBC Yorkshire Committee and their representative on Council. She is also Vice Convener of the Building Limes Forum. The tower of Holy Trinity, Challacombe, Devon: although initially rather bright, by 2004 (six years after completion), the render had started to tone down. Subsequently, despite some of the limewash weathering away, the render continues to perform well, creating a much drier internal environment.
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