We propose a numerical model for efficient design and optimization of a novel infrared (IR) detector array, fabricated on a single micro-electro-mechanical system (MEMS) membrane based on silicon on insulator (SOI) technology. The model is based on a finite element method (FEM) and is used to investigate the effect of heat transfer, at pixel scale, on the thermopile array’s responsivity and crosstalk performance. We show that optimal operational conditions can be achieved by modifying a combination of design elements, including the pixel size, the interpixel metal heatsinking tracks, as well as the insertion of air gaps between pixels. We find that the combined effect of copper heatsinking tracks and that of air gaps can reduce pixel crosstalk by 65% while increasing the responsivity by 6.4%. The model improves our understanding of the thermal effects in these devices, and can serve as a design tool for a growing number of low-cost industrial and consumer applications.