Our research shows that traditional lime harling and limewash do far more than define the appearance of Scottish architecture—they are essential to keeping wind-driven rain out. Historic buildings have survived for centuries, yet many are increasingly vulnerable to water penetration and decay making many of them unviable in a changing climate.

This interdisciplinary study explores why traditional lime finishes are important in protecting historic fabric during episodes of wind-driven rain. Scotland’s traditional buildings are not airtight and if the sum of infiltrating air on the windward elevation is greater than the exfiltrating on all the other elevations and the roof combined, then there will be an external/internal pressure difference.

Pressure difference is the corollary of decelerating wind speed at the wall face. Our experimental work highlighted two simple principles: any pressure difference above 40 Pa, significantly increases water penetration in conditions of wind driven rain. Secondly, a common industry standard of conserving is the ‘heritage joint’, meaning lime mortar pointing subsequently tamped back to produce a distressed finish. While the method decreases inundation it does not stop it. Only when full harl and limewash were applied, was the problem resolved.

The research also examined the evolution of Scottish architecture during the Long Eighteenth Century (1680–1833), when construction moved from robust broad-wall masonry towards the lighter forms of Neo-Classical architecture. Although culturally and aesthetically successful and widely adopted, these buildings incorporated thinner masonry generally and specifically under window openings. On plan and in section, walls become a series of substantial piers linked by insubstantial ties making them susceptible to structural cracking.

Once cracks develop, water ingress becomes a long-term problem, particularly where protective lime finishes have been removed or replaced with cement-based materials. Recent and ongoing research into Scottish building traditions has identified diversity of historic lime finishes, including sixteen distinct pointing styles. Harling and limewashing appear to have been commonplace from the early medieval period until the mid-nineteenth century, whereas no evidence has been found for the historic use of the heritage joint. The research concluded that there was a simple, previously well-understood vernacular building practice in Scotland that recognised that seamless buildings were less likely to leak in wet and windy conditions. Taken together, these findings suggest that current conservation practice should reconsider the authenticity and effectiveness of this now widespread approach and re-evaluate the protective role of complete lime finishes in safeguarding Scotland’s historic buildings.

My paper forms part of the ICMB25 series in UCL Open Environment, first submitted as an abstract for the 3rd International Conference on Moisture in Buildings (https://ukcmb.org/icmb25). I chose to submit my full paper to the UCLOE series because the journal welcomes and recognises the value of interdisciplinary research in building resilience. The journal offers an Open Access platform that is free to publish in and does not restrict the number of figures and tables nor do they charge for them.

Lime finishes: what's the point? Pressure difference in storm conditions in a changing climate by Tim Meek (The University of Stirling, UK), Kate North (The University of Stirling, UK), C. Douglas Moore (The University of Stirling, UK; Forest Research, Northern Research Station, Roslin, Midlothian, UK) and Jens-Arne Subke (The University of Stirling, UK) is part of the UCL Open Environment series, 3rd International Conference on Moisture in Buildings (ICMB25) and published in UCL Open Environment volume 8.

Tim is an apprentice trained, award winning bricklayer who, in his early twenties, attended night school then undertook a degree in archaeology at the University of Newcastle Upon Tyne, followed by a Master’s at the University of York and finally, in 2022, a PhD at the University of Stirling where he is currently furthering his research into the impact of lime finishes in pressure difference scenarios and writing the academic papers which are an outcome of his continuing research. He is also co-supervisor of Kate North’s (English Heritage) PhD research. Throughout his career he continued to work practically, pioneering the reintroduction of lime finishes, principally harling and limewashing to Scotland and crucially, this practical work was predicated on archaeological evidence rather than assumed values – so pracademic! Training the next generation of young builders and academics has been a core value of his working life and that training extends to precise project management, often on multiyear, complex, high commercial value projects. The engagement of a new generation of heritage professionals continues through the establishment and management of the Lime Finishes Group.

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