Interesting thread that never seems to ask and answer basic questions;
1. How hot does it get with the lights on and no ventilation? If it doesn't get hot enough to need exhausting, your first problem is adding enough insulation to change that.
2. I haven't seen/could have missed a clear description of what you're doing to maintain heat at night.
@jijiandfarmgang knows her shit and so does
@SnapsProvolone . Both have mentioned thermostats but I don't see that you've installed one. Why not?
The plants want a nice consistent environment within a fairly narrow temperature range- which are perfect parameters for the use of thermostats. When the room gets hot, a cooling thermostat should control an exhaust fan- which will only run when it gets too warm. No guesswork required. This will also keep daytime RH from getting out of control. If you're concerned about cold intake air flash freezing plants, aim the intake directly at the floor and put a fan on the same circuit that blows that air against a wall, mixing and preheating it.
At night, a heating thermostat should control both a heater AND a fan that will blow that heat around so it mixes the air in your room. This will help keep RH down at night.
Put your CO² burner on a CO² controller and let it do its thing if you want, although I'm with Jiji in that I don't think it will make much difference over venting. Fully sealed room HVAC is a long step up the expense curve and that's when that burner will do you the most good.