Sorry I feel you've totally underestimated the importance of co2. Co2 is added in a sealed room. Its a must, along with de/humidifiers and air cons.
The general guideline on vented systems, is the need to replace the air every 5 minutes.co2 is important .... but additional co2 is not needed for a few plants. there is more than enough in a normal sized room with the doors being opened very now and then.
co2 is important ... the size of a grow will determine the need to supplement or to use what is already in the grow environment. temperature also plays an important role in the uptake of supplemental co2 by plants - it needs to be nice and warm for a plant to really utilise co2.
co2 in a grow room, can come from the plants themselves, from the soil (the microbes and living things within) and from people. it is possible to buy a cheap co2 metre and it is amazing to monitor how much co2 comes from people in a room.
my poor use of 'sealed room' is probably confusing things a bit ... in a true sealed room, constant monitoring and adjustment of co2, moisture, heat and air movement is vital. these kind of setups though are quite elaborate and for big grows - basically commercial setups with high initial overheads. for a few plants, a recirculating system where no air is exhausted outside, is a better term. in small grows ... no additional co2 is needed for good healthy plant growth - only the air in the room from everyday use, is needed.
i have a feeling that the 5 min refresh rule comes from the marketing department of a hydro fan or carbon filter company.
the 5min rule also factored in the heat that hid lamps produced in a small area and the need to get rid of that heat quickly. even canfan have to make this concession and have a 'recirculating' rate as well as the standard 'exhaust' rate on their refresh calculator.
the use of heaters, aircons, humidifiers or dehumidifiers when exhausting the tent air outside is incredibly wasteful and expensive. i know, because i did it that way for years. it was only until a time came, when i could not vent outside due to stealth reasons, that i had to try something different and go with a recirculating (non-vented grow room).
there is a lot of information about non-vented grow spaces around. most of it comes from growers in cold climates, who can not exhaust outside, due to the cost of heating sub-zero air constantly, in order to vent outside - after all, you need to initially bring in air to exhaust it.
once I got used to this way of growing (which requires good environmental data gathering systems), even in moderate climate locations ... i will still use non-vented grow rooms for the amount of control i can get over an environment and the reduced hassle and costs.
if you get the chance and are only growing a few plants ... or if you have a grow situation where the climate or stealth requirements make it difficult to vent outside, why not give it a crack? from my experience, this makes growing heaps more fun, as it reduces most of the environmental concerns, which in turn, reduces pest problems and nutrient uptake issues.
it just makes things easier and cheaper.
. Cheers
hope you have a good day
feeling very welcomed 
