Sealed is better to keep the Co2 in and maintain levels. Entirely sealing a room is quite hard if you haven't planned way way in advance. You just have to release more Co2 in the air to account for the fact you are losing it. Luckily for us its relatively cheap. An entirely open area would require massive amounts of co2 poured on it from the top (heavier than air, so it floats down the plants as its released giving a nice cloud of goodiness) and would not be an effective environment to do this in. For one because you need air circulation. The Co2 and Air must be moved around the leaves if you really want to see a gain from it. A fan would quickly move this co2 out of the grow area to be hard to be brought back.
Entirely sealing a room is not required though. As long as you have an area that is mostly closed in for the gas to float around you should be okay. Account for these leaks in your calculations though. Any big gaping holes should be closed off as much as possible. Cooling your lamps down with the exhaust fan is necassary so account for this as well.
Your Co2 enriched environment will actually prefer the higher temps. I try to keep my garden at 80. My plants start stretching at 85 so I like to stay away from that as much as possible. I only use my Air Conditioner when I absolutely have to so not to pull any air unnecassarily out. As long as you stay within the 75-85(85 may be a lil high someone can correct me on this if they like) you should be great, actually preferred. Now 86+? No good. Your plants will remain healthy, even continue growing normal in a co2 environment, but they will see no benefit other than just healthly growth, no exponential growth. With Co2 you can raise into the 90's and plants be fine. Of course this is receiving an optimum 1500 PPM, lower that, lower the figures slightly.
So to answer your question, keep your room at 80 however possible. Pump in co2 however possible. If your room is mostly closed in but a lil open, just pump in more co2, your plants will really thank you for it