Well My 2 cents on that is you don't want CO2 at night anyway as it can stunt the growth. The plant uses O2 at night and does kind of a reverse process to make sugars a different way and helps it get a boost in the morning. That is really the only good reason not to run 24 ON from a general growth perspective. But all studies I have seen show 24 on done right is superior. I tend to use both. Like in summer I definitely try to bloom through August and September and if not I go 18 on and shut her down in the afternoon.
I have heard of people using dry ice for the CO2 but not for cooling.
CO2 is heavy and will fall down. So for best result s place it above the plants so the CO2 when it evaporates falls down to the plants. That is unless you have some good venting going on and then you need to work it with the venting...and that is the real problem with CO2.
To do it correctly you need to get the PPM real high. I can't remember I think above 900 PPM but basically almost dangerous to us. and you want the temps and humidity high too. So the right way to do it is in a sealed room with well insulated ducting on Air cooled hoods so you can keep the room sealed. then you have a "lung" or recirc room or ducting. you place a fan and run long hose all around or into a cooling space to cool the air and bring it back in. like the freon lines for the fridge. so you run this big hose all over that is not insulated and being cooled and then back into the room. then you can try to keep the air temps right by recirculating the coair but keeping it a sealed chamber. I don't know if that is easy to understand but there it is.
CO2 is difficult to do right. It requires a lot of extra expense. It does not get a lot more yield. If you are growing 4 plants you are way better off to just grow 5 then trying CO2. If you are growing 50 plants then CO2 can make a huge difference and is worth it.
You are like the second person this week to talk to me about using CO2 to solve a heat problem. I say add more vents. your idea is slightly better then just adding CO2 but still the problem is the ambient temps.
I would recommend getting some plywood and building a closed space in the attic. Then have an inlet vent from inside the house where it is likely cooler or running it to the edge of the roof for an inlet vent and then dumping the hot air elsewhere through a good hose out another vent. Growing in the attic is a Macgyver game, you are gonna have to get a bit fancy.
