Buffering Heat and Moisture: In Situ Performance of a Sisal-Fibre Earthen Plaster in a Mediterranean Climate

Earthen plasters are attracting interest as low-impact, vapour-open finishing systems, yet most evidence still derives from laboratory characterization rather than from full-scale exposure. Their application in real buildings is still constrained by regulatory and material-related barriers, as national regulatory frameworks remain uneven and existing guidelines provide limited support for fibre-reinforced earthen mixtures. Moreover, natural soils vary in composition, making standardization and quality control difficult. In Mediterranean climates, vapour-open materials can support passive moisture regulation. However, seasonal changes in solar exposure and rainfall make full-scale assessment under real operating conditions necessary. This study assesses the in situ hygrothermal behaviour of an experimental circular earthen plaster, formulated with marble-processing dust and sisal fibres, against a natural hydraulic lime (NHL) reference. Two near-identical hollow-clay-block masonry test boxes were built near Catania (Southern Italy, Köppen Csa) and monitored under free-running conditions across winter, transition and summer campaigns in 2026, recording surface temperatures, indoor air temperature, relative humidity and CO2. In winter, the earthen finish reduced the daily internal surface amplitude from 4.9 °C to 4.2 °C consistently on every paired monitoring day. During the transition period, surface differences were negligible, but relative humidity emerged as the discriminating variable, the earthen room remaining higher and more stable. In summer, the hygroscopic buffering became most evident, with the earthen room holding indoor humidity near 72% almost unchanged across a near-five-degree seasonal warming. A difference in solar absorptance between the two external finishes was identified and quantified, reducing the external surface peak of the earthen prototype by about 2 °C at maximum west-wall irradiance. The results support the use of full-scale monitoring for circular earthen plasters, advancing scientific understanding of circular earthen plasters under Mediterranean exposure and providing applied evidence for their performance-oriented use.