We assess the impact that furniture and contents (i.e. internal mass) have on zone peak cooling loads using a perimeter zone model in EnergyPlus across 5400 parametric simulation runs. The zone parameters were HVAC system type (overhead, underfloor, and thermally activated building system (TABS)), orientation, window to wall ratio, and building envelope mass. The internal mass parameters were the amount, area, and the material type used. We also evaluated a new internal mass modeling method, which models direct solar radiation on the internal mass surface, an effect that is missing in current methods. We show how each of these parameters affect peak cooling load, highlighting previously unpublished effects. Overall, adding internal mass changed peak cooling load by a median value of -2.28% (-5.45% and -0.67% lower and upper quartiles respectively) across the studied parameter space. Though the median is quite low, this study highlights the range of effects that internal mass can have on peak cooling loads depending on the parameters used, and the discussion highlights the lack of guidance on selecting reasonable values for internal mass parameters. Based on this we recommend conducting an experimental study to answer outstanding questions regarding improved specification of internal mass parameters.