You’d really need to do a CFD analyis using ANSYS or the like to get a feel for it, because the (humid) air inside the balloon will not be stagnant because of convection. The less well your envelope insulates or the larger the temperature differences, the more the air moves, and the less well you’ll be able to make use of the (stagnant) air inside the balloon as an insulator. If you’re using humid air, you’d like to keep the temperature of the material that envelops that above the dew point to avoid condensation.
You could try some back of the napkin calculations. Pick a constant inside and outside air temperature, and thickness and thermal conductivity of your envelope, then play with this equation: https://www.khanacademy.org/science/physics/thermodynamics/specific-heat-and-heat-transfer/a/what-is-thermal-conductivity
What is lost must be supplied by your heat source. Excluding extra heat loss from the influence of wind, I think this is a worst case scenario because in some places the air that is closer to the envelope inside of the balloon may be colder than that elsewhere (near the top of the balloon or close to the heat source) and it doesn’t take into account the extra insulation stagnant air inside the balloon may give, if you manage to get stagnant air inside the balloon.
I personally think this is a dead end. I think using a hydrogen balloon would be less of a dead end.