Application of High-Reflectivity Hydrophobic Nano Paints as a Retrofit Strategy to Enhance Buildings' Energy Performance in Cairo

Document Type

Article

Publication Date

2026

Abstract

In Egypt, the solar reflectivity of exterior walls drops from 72% on new surfaces to as low as 20% due to dust, pollution, and minimal rain, with no affordable, scalable retrofit solutions available. This study investigates, in the Egyptian context, whether zinc oxide (ZnO) nanoparticle-enhanced white acrylic paint can restore and sustain high wall reflectivity under prolonged UV and thermal exposure at a cost accessible to Egyptian households and contractors. Parametric energy simulation (DesignBuilder v6.1 / EnergyPlus v9.x) across a 100-run factorial matrix of window-to-wall ratio (WWR: 10%– 70%) and wall reflectivity (R: 10%–95%) for a Cairo reference building shows that upgrading from a degraded surface (R = 20%) to a ZnO-enhanced coating (R = 90%) reduces annual cooling energy by 10%–20% across the WWR 15%–35% range that characterises most of Egypt's buildings, with a turning point at WWR 40%–45% beyond which glazing-driven gains dominate. Four ZnO paint formulations (0, 1, 3, and 5 wt%) were prepared by co-precipitation, verified by XRD and SEM, applied to cement substrates, and weathered for 28 days (60 °C, 70% RH, UV-A 0.83 W/m²/nm). The 3 wt% formulation proved optimal: post-UV solar reflectance of 0.74 (91.4% retention), colour stability ΔE* = 2.1, a 25% reduction in thermal conductivity, confirming active UV-shielding of the acrylic binder, and a hydrophobic contact angle above 90°enabling passive self-cleaning. The 5 wt% formulation failed due to photocatalytic degradation of the binder, resulting in a 201% increase in thermal conductivity after UV exposure. For a reference building with a WWR of 20%, the retrofit costs 5,500–6,000 EGP, saves 1,065 kWh annually, and achieves payback within 2.6–5.7 years. The principal contribution is a complete, evidence-based specification that covers simulation boundary conditions, optimal formulation, characterisation data, and financial viability for a passive-cooling retrofit requiring no structural modifications, no imported materials, and no specialist skills, making it directly actionable across Egypt's existing building sector. Limitations include laboratory-scale rather than in-situ weathering and a single-zone building model; future work should address multi-zone archetypes, extended field exposure, and green ZnO synthesis routes. © 2026 by authors, all rights reserved.

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