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?

turn off the intake, and see what that does... if it requires all three fans to keep it cool you may be stuck with the only option of getting a humidifier...

...the only issue with humidity I've ever had was due to it being too high during flowering, it caused bud rot.
...never have I had an issue with humidity being too low....low is good, IMO. Come season change, your humidity will change.