What if my room gets too humid?

Humidity, Excerpted from Chapter Six, Indoor Marijuana Horticulture, The Indoor Bible

Humidity

Humidity is relative, that is, air holds different quantities of water at different temperatures. Relative humidity is the ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature. In other words, the hotter it is, the more moisture air can hold; the cooler it is, the less moisture air can hold. When the temperature in a grow room drops, the humidity climbs and moisture condenses. For example, an 800 cubic foot (10 x 10 x 8 feet) grow room will hold about 14 ounces of water when the temperature is 70 degrees F and relative humidity is at 100 percent. When the temperature is increased to 100 degrees F, the same room will hold 56 ounces of moisture at 100 percent relative humidity. That's four-times as much moisture! Where does this water go when the temperature drops? It condenses, just like dew condenses outdoors, onto the surface of plants and grow room walls.

A 10 x 10 x 8 foot (800 cubic feet) grow room can hold:

• 4 oz. of water at 32 degrees F.
• 7 oz. of water at 50 degrees F.
• 14 oz. of water at 70 degrees F.
• 18 oz. of water at 80 degrees F.
• 28 oz. of water at 90 degrees F.
• 56 oz. of water at 100 degrees F.

Relative humidity increases when the temperature drops at night. The more temperature variation, the greater the relative humidity variation. Supplemental heat or extra ventilation is often necessary at night if temperatures fluctuate more than 15 degrees F.

Cannabis Clue – The moisture holding capacity of air doubles with every 20o F increase in temperature.

A hygrometer measures relative humidity. Measuring and controlling humidity in a grow room is essential to a heavy harvest.

Wire a humidistat and a thermostat to a vent fan for grow room climate control.

Cannabis grows best when the relative humidity range is from 40 to 60 percent. As with temperature, consistent humidity promotes healthy, even growth. Relative humidity level affects the transpiration rate of the stomata. When humidity is high, water evaporates slowly. The stomata close, transpiration slows and so does plant growth. Water evaporates quickly into dryer air causing stomata to open, increasing transpiration, fluid flow and growth.

Warning! Air that holds 16 ounces of water at 75 degrees F will hold only 10 ounces at 60 degrees, causing 6 ounces of water to condense out of the air into the room.

Transpiration in arid conditions will be rapid only if there is enough water available for roots to draw in. If water is inadequate, stomata will close to protect the plant from dehydration, causing growth to slow.

Measuring Relative Humidity

Relative humidity control is an integral part of insect and fungus prevention and control. Humidity above 80 percent discourages spider mites but promotes fungus, root and stem rot. Humidity levels below 50 percent reduce the chances of fungus and rot.