Polymerizing a water-in-styrene high internal phase emulsion (HIPE) and subsequently expanding the polyHIPE through heating could present an environmentally friendly alternative to the synthesis of industrially expanded polystyrene foams where pentane is used as blowing agent. The polyHIPE has to have a closed-pore morphology to build up enough pressure during heating so that the polystyrene matrix expands before the water escapes. We found that the polyHIPE has to contain only small amounts of the crosslinker DVB to keep the Tg of the polystyrene matrix low and thus to ensure expandability. In addition, the water volume fraction ϕdisp should be as high as possible to obtain polymer foams with low densities. To find a HIPE with a suitable composition for expansion, we systematically varied ϕdisp, the DVB concentration cDVB, and the surfactant concentration csurf. We investigated each composition by studying the stability of the HIPE before polymerization, by analyzing the morphology of the polyHIPEs with scanning electron microscopy, and by examining the water release rates of the polyHIPEs. To have a highly porous polystyrene foam, ϕdisp must be ≥ 74 vol%. However, no suitable composition was found with this high ϕdisp which is why we suggest that a stepwise, systematic reduction of ϕdisp at the expense of the porosity could be a solution to this dilemma.