Bio‐Stabilised Earth and the Sustainability of the Built Environment: Experimental Evidence of Passive Environmental Moderation

In a context of increasing pressure on natural resources and the need for lower‐impact, circular construction materials, bio‐stabilised earth is receiving renewed attention. This study evaluates the environmental response of a full‐scale prototype built with locally sourced earth, tannin and fique fibres, monitored for 41 days in Madrid under a permanently naturally ventilated, non‐hermetic semi‐outdoor configuration. Rather than demonstrating the known thermal‐mass or hygroscopic behaviour of earth, the study assesses whether measurable hygrothermal moderation remains observable at construction scale under strong indoor‐outdoor coupling. Temperature and relative humidity were the primary performance variables, while particulate matter and CO 2 were complementary observations. Across 37 valid paired days with at least 95% temporal coverage, the mean daily P5‐P95 air‐temperature range was 7.70°C outdoors and 3.83°C indoors, yielding a mean normalised indoor/outdoor amplitude ratio of 0.510 (95% bootstrap CI: 0.486–0.539), equivalent to 49.0% lower robust daily variability indoors. For relative humidity, the corresponding ranges were 28.73 and 17.03 percentage points, with a mean ratio of 0.593 (95% CI: 0.537–0.651), equivalent to 40.7% lower variability. Paired Wilcoxon tests were significant for both variables ( p < 0.001). PM concentrations were only slightly lower indoors and are not interpreted as evidence of filtration. Aranet4 CO 2 series showed strong concurrent indoor‐outdoor variation ( r = 0.936), supporting strong air coupling rather than pollutant attenuation. These results provide case‐specific system‐level evidence of hygrothermal moderation without attributing the response uniquely to bio‐stabilising additives or extrapolating it to comfort, energy savings, long‐term durability, life‐cycle advantage or quantified SDG performance.