As the panda film has great light diffusion and reflectivity compared to the foil insulation and this being a sealed grow room I will 100% put it as the last layer towards the inside of the room.
Now my only real question is whether plastic wrap will be needed between the foam board and faced fiberglass... I do know that this will create a double vapor barrier or actually kind of a triple vapor barrier as the insulation is faced, plus the plastic, than later the panda on top.
Don't want to create trapped moisture INBETWEEN any of the insulation layers. The idea is to trap the moisture in the room completely and regulate through exhaust and or dehumidifier. I need the sealed room due the fact I will be adding CO2 to it.