||The properties of polymer foams are tightly linked to the porous structure of the material: two of the most decisive parameters are the interconnectivity of the pores and the thickness of the pore walls. Despite the vital importance of these parameters, a deep understanding of the processes that control the wall thickness and the pore opening is still lacking. We tackle these questions by studying monodisperse, highly ordered polymer foams which are generated via emulsion templating using microfluidic lab-on-a-chip techniques. We explore the influence of different processing parameters, and we show that the most crucial parameter is the locus where the polymerization is initiated. If initiation starts within the continuous monomer matrix, the morphology of the liquid template is “frozen in” with pore openings arising where neighboring drops are separated by thin films. However, if the locus of initiation is at the interface, not only do the pores remain closed, but we evidence a hitherto unexplained mechanism which leads to an osmotically driven redistribution of monomer in the walls during polymerization. This changes dramatically the pore morphology (polyhedral pores with thick walls) and therefore the final material properties opening the pathway to new applications of low weight or acoustic bandgap materials.