Once you've installed your air conditioners and co2 systems, you'll need humidity control to complete the trifecta of climate control in order to perfectly dial in your grow room.
Most serious growers will need both humidifiers (for propagation & vegetative growth) and dehumidifiers (for the bloom, maturation & ripening phases). It is essential to maintain precise control over your environment and keep your relative humidity, temperature & co2 at proper levels in relation to one another. Once you're able to achieve perfect VPD (Vapor Pressure Deficit) levels, you'll take your grow room to the next level. Yields will increase and crop cycles will be shortened significantly. Your plants will be much more naturally resistant to bugs, mites, mold, mildew & stress.
Inadequate humidity control can lead to a list of grow room problems. Mold, mildew and disease such as Botrytis, powdery mildew, black mold and bud rot are common issues related to excess humidity in a growroom. This also helps to reduce pests and plant dress caused by these bugs. When planting seeds, propagating clones and in the early vegetative phase, plants will require elevated levels of high humidity which in some climates may require a humidifier. Optimal humidity levels are upwards of 90% for propagation so you'll need to choose a humidifier that can handle the needs of your grow. Ambient temperature and humidity levels and your environmental climate should always be considered. Make certain you are comparing apples to apples when shopping for humidity controllers and dehumidifiers as some manufacturers use AHAM (Association of Home Appliance Manufacturers) rating standards and others do not. AHAM rated units are tested in 80 degree temperature and 60% relative humidity chamber for 24 hours and then rated based on their performance. Many manufacturers will test their units in an environment with much higher RH & Temps which will yield a much higher output of the of the very same machine. These manufacturers will then list a unit that performs for example 100 pints per day under AHAM standards as a 200 pint per day output (after testing their machine under more humid and warmer conditions). Electrical efficiency is also a major consideration when building out any facility, especially larger commercial facilities where scale and attention to budgets are particularly important. Efficiency varies wildly with some units removing nearly 9 pints per kilowatt hour whereas lower end units in some cases can produce only 1-2 pints per kwh. Quest overhead dehumidifiers or portable dehumidifiers paired with Quest F9 Air Movers and Hepa Air Scrubbers will make for a perfect environment in a commercial grow. Hobbyists and smaller scale commercial growers may consider a unit such as Ideal-Air or Aprilaire for less industrial needs.