Humidity Issue

I started my first grow in a stealth pc grow box and it contains 2x 23w 6500k CFL bulbs and 3 pc fans (1 intake, 1 movement, 1 exhaust).
I am using a digital thermometer/humidity meter and when the lights are on (12/12) the humidity in the case goes below 20% and meter reads low.
I added a cup of water to the case hoping that would add moisture and increase the humidity in the case, but it hasn't.
How can I increase the humidity in my pc grow box?

Never tried a pc box before but im sure if ya put a wet rag in there it would help some ,just my 2 cent