Growing in Australia, I struggle to keep my temps at 79°F. I’m thinking of introducing an evaporative cooler. How will this affect the relative humidity in my room and is it a viable option compared to air conditioning?

Evaporative cooling can be highly effective, however, the amount of cooling that can be achieved with evaporative methods is not unlimited. It’s also linked to temperature and humidity of the incoming air. As water evaporates into the air, energy is lost which causes the air temperature to fall. The latent heat of evaporation of water is 2,257 kilojoules per kilogram, meaning that for each liter of water evaporated, 2,257 kilojoules of energy is removed from the growing environment as heat. The cooled air is then circulated through the plants, lowering temperatures in the environment until it is vented out the other side. To work out how much cooling is possible in a well ventilated growing area, two variables are important: the dry-bulb and wet-bulb temperatures. The normal air temperature we can measure with a standard thermometer placed in the ventilation stream of the growing area is the dry-bulb temperature. The wet-bulb temperature is more complex but represents the lowest temperature you can achieve by evaporation of water only, and this is linked to the moisture content or humidity of the incoming air. The wet-bulb temperature tells us how much cooling is possible given the temperature and relative humidity level of the incoming air. The higher the relative humidity of the air, the less cooling is possible as this lowers the amount of evaporation that can occur. Wet-bulb temperatures these days can be easily measured onsite using electronic humidity meters which can display wet-bulb temperatures. Growers who don’t have electronic displays can still work out the cooling potential of evaporation by the slightly more old-fashioned way of using psychometric charts. These charts allow wet-bulb temperature to be calculated based on dry bulb or normal air temperature and the relative humidity which can be measured with a standard RH meter placed in the air stream. Generally, air in the growing area can be cooled and held to a wet-bulb temperature of 3˚F.

It also important to remember the cooled air will gather heat as it circulates in the growing area. Heat accumulation depends on factors such as how far the cooled air travels inside the growing area and heat output from lamps and other sources. It is possible for the temperature build-up inside the indoor garden to reheat the circulating air to greater than that of the outside environment, so correct calculations of both the amount of evaporative cooling and amount of air to be vented in and out need to be done correctly. Ventilation is an essential component of evaporative cooling. As water evaporates, cooling the air, humidity goes up. Since higher humidity can create problems for plant growth by slowing transpiration and increasing the occurrence of disease, humid air needs to be vented out at a sufficient rate to keep relative humidity down below 89-85 percent.

In climates with high outdoor humidity, evaporative cooling can’t be used. In this situation, air conditioning units are a good alternative and can also lower temperatures much more when needed. Air conditioning does produce relatively dry air, though, so it may need to be re-humidified with either humidifier units or wet pads/open pans of water before being circulated over plants.