Lime finishes: what's the point? Pressure difference in storm conditions in a changing climate

Abstract

Climate change is driving international heritage to reconsider the conservation strategies applied to traditional historic buildings. To date, the preferred repair method for rubble stone buildings in the UK has been pointing, and the style employed is the heritage joint: a recessed joint that highlights the profile of the stone with aggregates exposed. This paper challenges that conservation approach, by demonstrating that a bare stone method leaves historic fabric in Scotland vulnerable to water penetration, particularly in the north and west regions where wind-driven rain predominates. In addition, we argue that the heritage joint lacks historic authenticity, and that the accurate presentation of architecture is related to the original functionality of the building system. By turning to the archaeological record, we demonstrate that pre-industrial Scottish vernacular buildings were originally completely lime-coated, and that this was an effective way of managing moisture. By the 18th century, changing tastes expressed a preference for bare stone and ultimately this led to finishes being stripped and the joints between stones being pointed in many different styles, few of which are employed within the heritage sector. The strong winds experienced in storm conditions create external/internal pressure differences on windward elevations. The rain that accompanies wind is attracted into the internal space in these conditions where wall widths diminish, and where there is no external finish. In an innovative experiment, the actualistic pressure differences observed in storm conditions are created using a purpose-built airtight chamber and these pressure differences are accompanied by simulated rainfall. The results of the experiment show that pressure difference is a significant contributor to water penetration at the window embrasure. Crucially, the experiment demonstrates that completely lime coating rubble stone buildings moderates this ingress, while the heritage joint is the least effective barrier to water. The baseline for conserving and improving historic rubble building fabric is considering complete lime finishes as vital tools in building resilience.