Sounds like you're painting a jail cell and warming it with a ceramic heater. IF UNDISTURBED, the vapor may settle so that the denser gases are at the bottom of the cell and lighter gases at the top. However, any movements (such as you moving through the room) will stir it all up again. AND temperature variations will cause air movement and stir things up also. So I don't believe any layering effect is the problem here.
You state you're using a odor free epoxy resin. I have no idea what chemicals are in your particular brand but many of those odor free ones use mineral / vegetable oils, small amounts of petroleum distillates and pigments. And if you're using a a hardener, they might contain amines, phenols and formaldehyde (the latter is notorious for causing headaches, vomiting, irritation of throat, nose, etc). I doubt the pigments are the root cause of your headaches but any of the rest of the chemicals are fair game. Can you elaborate on brand? and what's this about butane?
As to ventilation, the general idea is you want to bring in fresh air where you breathe and vent air away from where you breathe. If you're standing up and painting the floor with a roller, an inlet vent on the wall or ceiling and an exhaust fan on the floor might be best. But of course, you want to paint your entire cell right? Not just the floor. So another consideration is air turnover rate.
Although to properly design a ventilation system requires knowledge of the chemicals in the vapor phase, as a rough estimate you could use the standard 15-30 turnovers per hr (CPH) for noxious fumes. Let's go with 30 for the moment.
you can estimate Q (cubic feet / min.. cfm) required by this equation.
Q = CPH * V / 60 = 30 * (2.4*2.4*2.0 m^3)*(3.2ft/m)^3/60 = 230 cfm
Most of the common bathroom exhaust fans are about 1/4 to 1/2 of that capacity so, you could try 2 bathroom exhaust fans placed on either side of your cell near the floor with an inlet near the ceiling or 1 high capacity fan near the floor. I suspect that will help tremendously.
as to the temperature effects bringing in cold air... Your major concern right now is ventilation. You don't want to harm yourself right? IF after correcting this the temperature is too cold, add another space heater.