Bridging indoor and outdoor microclimates: a multiscale co-simulation framework for the thermal performance of earthen buildings

Abstract

As global temperatures rise and urban warming intensifies, sustainable building practices are critical for reducing energy consumption and improving thermal comfort. While modern construction has relied heavily on energy-intensive materials like concrete and steel, traditional earthen construction offers promising passive cooling strategies that could reduce dependence on mechanical air conditioning systems. Earthen materials possess hygroscopic properties enabling combined moisture buffering and thermal mass performance that provide substantial energy-saving benefits, yet previous research has largely focused on indoor performance whilst neglecting the crucial interplay between building energy consumption and surrounding urban microclimate dynamics.

This thesis bridges this gap by developing a comprehensive coupled simulation framework that connects indoor and outdoor microclimates to quantify real-time bidirectional influences: how urban environments impact building energy performance, and conversely, how HVAC waste heat from conditioned buildings affects neighbouring structures and urban canyon thermal conditions. A novel quasi-dynamic co-simulation methodology integrates EnergyPlus Combined Heat and Moisture Transfer (HAMT) algorithm with OpenFOAM urbanMicroclimateFoam CFD solver to accurately model hygrothermal performance of earthen materials across spatial scales from material pores to urban canyons. Validation through inter-model comparison demonstrates that both models correctly capture complex time-dependent building thermal behaviour and urban microclimate phenomena including solar gains and heat flux exchange through building envelopes.

Application of the coupled framework to rammed earth construction across hot arid (Algiers) and cold arid (Fresno, California) climates reveals substantial performance benefits alongside important climate-specific and position-dependent constraints. Rammed earth achieves 16-24% summer cooling energy savings compared to concrete construction across all building positions, with this reduction translating directly to equivalent decreases in HVAC waste heat rejected into urban canyons. Furthermore, rammed earth demonstrates superior thermal buffering capacity with 78% attenuation of thermal stress under extreme inter-building thermal coupling scenarios compared to concrete's 40-50%, thus preventing dangerous positive feedback cascades where accumulated waste heat drives escalating neighbourhood-scale energy demands. However, the research also reveals asymmetric seasonal performance where rammed earth incurs 15-20% winter heating energy deficit in sheltered interior positions with limited solar access. This behaviour where buildings simultaneously exhibit higher mean radiant temperatures yet require greater heating energy results from hygroscopic moisture cycling that consumes latent heat during critical morning heating demand periods whilst delivering thermal energy during mistimed afternoon-evening periods. Consequently, optimal deployment requires careful climate-specific, position-specific, and season-specific assessment, with strongest benefits demonstrated in cooling-dominated climates exceeding 70% annual cooling loads, at windward and leeward building positions with full solar exposure, and in extremely dry conditions where hygroscopic moisture buffering elevates indoor relative humidity from inadequate 39-44% to comfortable 47-70% ranges.

Critically, the research reveals how urban microclimate effects fundamentally alter building thermal boundary conditions in ways that conventional standalone modelling approaches cannot capture. Detailed analysis of convective heat transfer coefficients demonstrates coupled simulations consistently predicting CHTC values 2-3 times higher than standalone simulations, with particularly dramatic differences at leeward Building-E where coupled values maintain approximately 19-20 W/m²K compared to highly variable solo values fluctuating between 1-14 W/m²K. These enhanced convective cooling effects, combined with canyon air temperature elevations of 1-3°C and position-dependent aerodynamic phenomena including flow acceleration and vortex formation, establish compelling evidence that conventional modelling treating buildings as isolated entities systematically misrepresents urban thermal dynamics. These findings establish compelling evidence that conventional modelling approaches treating buildings as isolated entities fail to capture urban thermal dynamics, thereby potentially leading to oversized HVAC systems, inaccurate performance predictions, and missed opportunities for passive cooling strategies in dense urban contexts.

This thesis contributes both methodological innovations through its validated coupled simulation framework enabling accurate assessment of passive material strategies in realistic urban contexts, and practical insights supporting earthen material deployment for climate change mitigation. The demonstrated 10-14% net annual energy savings in optimal hot arid climates, combined with 40-60% whole-life carbon reductions when accounting for embodied and operational carbon, establishes earthen construction as a technically viable pathway toward building sector decarbonisation. Moreover, the framework's quantification of urban heat island mitigation potential where material-based anthropogenic heat reductions represent measurable contributions particularly when combined with complementary cool surfaces and urban vegetation strategies demonstrates that building material selection constitutes an underutilised yet effective intervention addressing heat generation directly at its source.

By bridging indoor and outdoor microclimates through multiscale simulation, this work establishes essential foundations for integrated approaches to building design and urban planning that recognise buildings not as isolated entities but as dynamic components of complex urban thermal ecosystems requiring coordinated passive and active strategies to achieve substantial climate change mitigation and urban heat resilience objectives.