This is a article in growers edge i found i hope it helps
To Vent or Not To Vent?by Gaby Bronsztein
All things considered, the author thinks a well-equipped closed system that's vented on an as-needed basis creates the best indoor-growing environment for plants
Indoor growers are trying to maximize crop growth, just like all growers. Providing plants with optimal temperature and humidity as well as elevated levels of CO2 can have a positive effect on crop yield and quality. For example, the addition of CO2 into a garden has shown to increase yields upward of 15% and speeds up plant maturation time.
I often talk with people convinced that plants need fresh air to grow. It's true that plants need what fresh air delivers- temperature control, humidity control and the addition of CO2. But it's not the case that these things are provided only but the addition of fresh air. How do I know? Plants are grown in the space station and space shuttles. Of course, there's no air in space and, thus, no chance for air exchange. The climate inside space vehicles is controlled using techniques similar to those used by growers raising plants in sealed rooms.
Another myth is that air gets stale. This is also incorrect. Air doesn't get stale; it gets polluted. And even in an indoor garden we have decaying matter such as dead leaves or organic fertilizers that are breaking down. This can create odors that are vented to the outside in open systems, but we can easily take care of the pollutants with the use of a carbon filter used as a scrubber to clean the air.
Let me elaborate about the differences between open and closed gardens. To control the growing climate inside an open system, ventilation fans are used to introduce fresh air and exhaust warm or humid air, helping control temperature and humidity while also maintaining minimal levels of CO2. Although this is the most common method of climate control, it comes with problems. Most important is that your limiting factor in controlling your indoor climate is the climate outside. Consequently, you'll find it difficult to cool your room on a hot day or control humidity when it's humid or raining outside.
In vented gardens, air exchange sometimes makes it difficult and costly to maintain high CO2 levels. If your garden does not require regular ventilation to control temperature, CO2 levels could potentially drop to levels that could stunt growth and limit crop production. In addition, controlling the climate can also be challenging. For example, if you live in a cool, humid climate, you can overcool your room trying to lower the humidity. The opposite is true in a hot, humid climate- you could be humidifying your room while trying to cool it. It's like playing tug of war and can be quite frustrating. Generally, it is most difficult in winter and summer.
With closed-system gardens, if the equipment is sized properly, you are able to do something that is virtually impossible with open systems: you can set climate set points. Also, lack of ventilation means that it's easy to maintain high average levels of CO2 inside closed systems. Of course, CO2 injection with a monitored system is essential so that CO2 levels don't drop.
You may be wondering why anyone would grow in an open system. Well, here is the rest of the story. Closed systems also have disadvantages. One big one is the increased cost of setup. Air conditioning and dehumidifiers are significantly more expensive than vent fans, and they also use significantly more electricity. How do we overcome these issues?
When setting up a closed system, it is essential that you cool your lights either with air- or water-cooled fixtures. (If you're using air-cooled lights, these draw air from outside the garden through the light and back outside, thus cooling the lamp without venting the grow room.) This can greatly cut the cost of electricity used to run your air conditioning unit. Air cooling can remove up to 50% of the heat with a small fan that uses little power. Water-cooled lights (very new technology) can remove up to 90% of the heat with the use of recirculation water and a small pump. This can significantly reduce electrical consumption and also save you money by allowing you to purchase a smaller air conditioning unit. Dehumidification is not so simple, however. Dehumidifiers use a fair amount of electricity but are the only way to control humidity in a closed system.
I have found that some growers have had to install multiple dehumidifiers in order to control humidity, especially when lights are off. The reason for this is simple: humidity is relative to temperature. This means when air is cooler it holds less moisture, thereby increasing relative humidity. This happens quite quickly when lights shut down, and the end result is the dehumidifier can take too long to get the job done. Another reason why the dehumidifier has a harder time controlling the humidity when the lights are off is that the air conditioning was also dehumidifying the air. When the temperature dropped, the air conditioning stopped cooling as well as dehumidifying. And finally, dehumidifiers work best above 70 F. At cooler temperatures, a dehumidifier spends less time doing its job.
If you have a single dehumidifier and you're still having trouble controlling humidity, consider three options. First, heat the garden so that day and night time temperatures are close. Second, add another dehumidifier. Last, if humidity is higher inside than outside, briefly vent the garden to remove the bulk of the humidity, then stop venting and close the system again. This should be done with the help of a thermostat-humistat combination unit that will only turn the fans on if you break temperature or humidity set points that are set higher than your dehumidifier or air conditioning set points.
So again I ask the question: To vent or not to vent? In my opinion, the best indoor environment for plants is created by using a hybrid of the open and closed methods. Try not to ventilate the room to control indoor climate, except when necessary. I believe that this method allows the best of both worlds. Having a garden that is sealed allows you to control the climate more precisely and allows you to easily enrich CO2, while the use of ventilation during extreme circumstances can help keep electricity and equipment costs down.
