Kushite mud-brick thermal mass: translating vernacular principles into evidence-based contemporary building envelope design parameters for hot-arid climates

Abstract For over 3,500 years, the massive mud-brick walls of the Western Deffufa in Kerma, Sudan, have moderated interior temperatures in one of Africa's most extreme hot-arid environments without mechanical cooling. This study quantifies the underlying thermophysical principles using DesignBuilder/EnergyPlus simulation across five wall-thickness scenarios (30, 60, 100, 150 cm mud-brick, plus a 30 cm concrete baseline) during peak summer and winter weeks, evaluated against ASHRAE 55 comfort standards. The results show a progressive, nonlinear improvement in thermal mass performance with increasing wall thickness. The 150 cm Kushite wall achieved a time lag of approximately 11 h and a decrement factor of 0.20, attenuating peak indoor temperature by 11 °C (32 °C indoors vs. 43 °C outdoors)—a 57% improvement over the concrete baseline. A notable thermal inversion was observed at 100 cm wall thickness, where wall heat gain shifted from positive to negative (− 6 W/m2), intensifying to − 21 W/m2 at 150 cm. This behaviour, referred to here as the ‘Deffufa Effect’, reflects a transition from passive resistance to enhanced thermal buffering, driven by the volumetric heat capacity of Kushite mud-brick (ρc = 1,710 kJ/m3·K) and providing, under the unoccupied conditions simulated, a passive cooling benefit during peak afternoon hours (13:00–18:00). A three-stage Translation Framework converts these findings into material-specific envelope design parameters. Equivalent thicknesses were derived for reinforced concrete (45–62 cm), insulated thermal brick (30–42 cm), and compressed earth block (35–48 cm), suggesting that aspects of Kushite thermal mass performance may be approximated in contemporary construction, supporting further exploration of vernacular design principles for energy efficiency.