Purpose <p>This study investigates how vertical clearance and fin spacing influence the thermal performance of LED grow light heat sinks, identifying thermal limits that constrain layer arrangement and overall volume utilization in indoor vertical farming applications.</p> Methods <p>A steady-state computational model was developed in ANSYS Fluent to simulate natural convection and radiation from a grow light heat sink. The model was validated using experimental data from thermocouples and infrared imaging. Simulations examined varying vertical clearance (6.35 ≤ H ≤ 127 mm), fin spacings (4.87 ≤ S ≤ 9.9 mm), and heat inputs (78 ≤ Q ≤ 325 W).</p> Results <p>Thermal plume analysis revealed that larger H promotes stronger air velocity (up to 0.43 m s⁻¹) and better thermal mixing. Both LED junction temperature (T_j,max) and thermal resistance (R_th) increased exponentially as H decreased, with minimal benefit beyond 50 mm clearance. Fin spacing influenced air velocity and mixing, with an optimal fin count improving thermal performance without restricting airflow. Additionally, a comparison between the numerical model and experimental data showed a maximum temperature deviation of 6.37%, indicating strong agreement.</p> Conclusion <p>The study provides predictive models for T_j,max and R_th as functions of nondimensionalized clearance (H*) and Q, and a Nusselt-Rayleigh correlation (Nu_H = 0.0354 Ra_H^0.4112, <i>R</i>² = 0.9956). Design insights include providing recommended relative clearance of 0.55 at 325 W, which reduces layer height by 77 mm (7% for a 110 cm layer height) while maintaining safe junction temperatures. These findings offer practical guidance for designing compact, thermally efficient IVF systems.</p>