Yes you can use a sealed room with CO2 and absolutely no air exchange. That is what I and many other people use. If I cut off CO2 at 1500ppm when the lights go off, ppm will go down to about 900 or 1000 by the morning but it never drops below that for us. Its nice to lower the ppm at night, so if you could start to drop temps and turn CO2 off an hour before lights went off that would probably be ideal but I dont think its necessary.

A recirculating air conditioner is essential for this to operate correctly (one that does not remove any air from the room). There is a very fine line between achieving the temps needed for proper CO2 usage and getting things too hot and stressing the plants. An environment controller is a must.

I use an IGS-220 which allows me to set temp levels. When things get above 85, A/C kicks in. At night, my temps are set at 15 degrees lower then in the day. The A/C only needs to turn on a few times at night to keep this temp (sometimes not at all). Dehumidifier keeps things dry at night, the A/C keeps things dry in the day (they do the same thing, but an A/C blows away the hot air). The IGS is nice because it allows you to change differentials so you can change the amount of temperature change needed before kicking on the A/C, so if your A/C keeps going on all the time for short periods of time, you can increase the differential so that the A/C runs less often for longer but achieves nearly the same results. It also allows you to look at hourly temps/humidity/co2 levels, etc.