Passive radiative cooling offers energy-free sub-ambient cooling; however, its main challenge is achieving low solar absorption (visible − NIR) while maintaining strong thermal emission in the mid-infrared atmospheric window. In this work, we numerically investigate a planar porous anodic alumina (PAA)/Al bilayer as a spectrally selective emitter for daytime radiative cooling. The optical response of the PAA layer is modeled using an effective-medium approach based on the Maxwell–Garnett (MG) formalism, allowing systematic tuning of porosity. The simulations are conducted using COMSOL software with a highly refined mesh to ensure accuracy. The study demonstrates the influence of PAA thickness, Al thickness, and porosity on thermal emissivity in the mid-infrared range. Furthermore, field intensity, power dissipation, and penetration-depth analyses are consistent with mid-infrared spectral selectivity originating primarily from intrinsic phonon absorption within the Reststrahlen band of alumina, modulated by wavelength-dependent interference effects. At an incident angle of 35◦, where the simulated angular emissivity profile exhibits a local maximum for TM polarization, the PAA/Al bilayer achieves an average emissivity of 0.982 within the atmospheric window (8 − 13 μm) for a porosity of 30%. Moreover, the designed structure shows high PRC performance under normal atmospheric conditions, achieving a maximum temperature reduction of up to 8.3◦C below ambient temperature. Also, at thermal equilibrium, it delivers an anticipated net cooling power of 77.36W/m2. The PAA-based approach proposed here provides a promising route for producing low-cost, efficient radiative coolers at large scales for practical energy conservation