Reduced overwatering, moisture soakers, creating holes for air, using a polyethylene insulator, restricting the plant density, etc are some of the most cost-effective ways of decreasing the humidity of your grow tent.
Apart from these ways, you can create air holes, eliminate drains or storage areas, use absorbing soils, and other methods to manage humidity in grow tents. You can reduce the humidity in a grow tent using a dehumidifier or an air conditioner, but you can also reduce it by removing standing water and decreasing the density of plants.
Since temperature and humidity go hand-in-hand, using an air conditioner to reduce the temperature within the tent will reduce humidity. Bringing fresh air inside the tent from the outside may help reduce the humidity, as long as you are not living in a particularly humidity-rich area. If you are already running a fan at 11, you may want to also consider running an extra fan inside your grow tent to help circulate air and remove excess moisture.
Generally, however, you are better off placing an exhaust fan near the top of your tent, as this is where hot air collects. Any good grow tent setup should have an exhaust fan that helps to push the air outside, and another ventilation hole can let cool air into the tent. In veg, you may want to open your tents fly flaps to let out the air at the top of your tent, and that new air will circulate.
You can set up fans and exhaust fans to provide adequate ventilation and circulation of air within your tent. If your outside temperature is lower than the humidity target within your tent, then one of the first things you will want to do is to raise your tents exhaust fan speed to make sure that air rotates at least once a minute. If the air in your tent is wetter outside the tent than your target humidity level, you need to get a dehumidifier.
If your grow room humidity is too high, your plants cannot transpire and they hold on to any water they have taken in. If the grow rooms humidity is too low, your plants will become drier because they will be trying to transpire to make up for the difference.
Also, ideal temperature and humidity levels vary depending on what stage the plants are growing. Different levels of humidity are needed in different stages of the plants life to ensure that the seeds, vegetables, and flowers are growing properly. For instance, when temperatures increase, air is more likely to retain water, and plants do not transpire as much if both temperatures and humidity levels are higher, and growth will be stunted.
Reducing the humidity level helps remove the excess humidity in the air, creating a more ideal growing environment for your plants. Plants will still need to absorb moisture via their leaves, so you will still want a relatively high level of humidity (60%-70%) for the early parts of your growing cycle. High humidity levels are fine for vegging, but in bloom, particularly in the late stages of bloom, you will want to dramatically lower your humidity levels.
With that remaining 95% going back into the air, it is essential that your grow tents relative humidity stays consistent and in the correct levels. When relative humidity is too high within your grow tent, plants will often soak up too much water in their systems via their leaves. If you have an open surface of the water within a grow tent, this becomes another moisture source for the air and increases the humidity.
When trying to raise the humidity inside the grow tent, many times, people will set up bowls or trays with water, which then can be vaporized to provide humidity for plants. The water is then fed to the humidity controller, which floods the room with humidity, thus increasing humidity. If relative humidity (RH) is brought to a suitable level, water within the leaves is encouraged to transpire due to low humidity and pressure.
You can set your desired RH and a dehumidifier will make sure that your tent stays at this level consistently. Most growers who use a grow tent will not need to use dehumidifiers, but there are cases when your plants get really bushy/crowded, and you might have trouble keeping humidity levels low. If you have too many plants in a grow tent, it could dramatically increase water vapor in the space, since it would become too crowded.
An easy way to decrease the humidity in your grow tent is to ensure that you are not leaving standing water from the drains into your planters. Adding a dehumidifier, opening some vents on the ventilation system, and getting rid of excess water in your tent are all great ways to lower your humidity.
By just setting up the ventilation system right from the get-go, you should never have too much trouble with moisture, provided that you are not over-watering and leaving excess runoff sitting inside your tent. Adding ventilation vents in your tent or rooms can help to bring in the cooler air outside, which can help to regulate your interior temperatures. Humid air is heavier than dry air, so you may want to consider setting up exhaust fans at lower levels if moisture is a big problem but temperatures are not.
With larger tents, or to prevent a suck-in of your tent, you will want to have a second, ducted fan blowing cool air back into your tent, replacing air that is being sucked in. You can also use fans to push cooler air into the interior of the growing space, something that commercially grown facilities are known for using. If you are dealing with both high humidity levels and high-temperature levels inside a grow tent, then air conditioning is the best option.
While you cannot control humidity coming in from the outside of the grow house, you can keep it out using insulation. The only way around this is to completely insulate the tent, but the smallest of gaps in your insulation can let that humidity get in.
Different varieties of plants are affected differently, so you need to manage the humidity according to the particular variety that you are growing. Different plants need varying levels of humidity — seedlings, flowering plants — so there will be different times when the need arises to raise or lower humidity levels.