Mr. B:
I assume you are fermenting in some sort of fermenter inside the Igloo cube, so you have the fermenter in a water bath in the Igloo. If the fermenter is a glass carboy, and your water bath termperature is fairly stable, then you beer temperature will be very close to you water temperature. This is due to the fact that the glass is a good heat conductor, and you have good thermal contact between the beer and the water via the glass carboy. During the primary (active) phase of fermentation, when the yeast is generating maximum heat, the fermenting wort is being well stirred by the CO2 off-gassing so you have efficient heat transfer to the cooling water, and you have convection currents in the wort as well as the wort at the carboy surface is cooled, becomes denser than the warm wort and sinks to the bottom of the carboy. During the secondary (slow)fermentation phase, little heat is generated so the temperature of the beer is the same as the cooling water.
The more water you have in the igloo, the better you can manage the fermenting wort temperature due to more thermal contact and more water to absorb the heat of fermentation. If you are using a plastic fermenter, you will have a bit less thermal conductance between the wort and water due to plastics being better insulators than glass, but your wort temp probably won't be much different higher despite this in the primary fermentation phase.
And for the little that it's worth, I am a chemist, but a physicist or chem engineer would give you a better answer with lots of cool math that I could never understand...but the basic answer is the same.