Humidity

In the overall growing equation, how heavy of a role does humidity play? I am having some low humidity issues in my mini grow box ( 20x21x28 ) with 4 23W 6500k CFLs. My temp is hovering around 79 with a circulation fan and a 4" exhaust fan. I currently have negative case pressure which I may like to retain if I can find a small enough carbon filter to mitigate flowering aroma. My home humidity is at 36% and even with a small desktop humidifier inside the box I am not getting too much higher. I am receiving an full spectrum LED tomorrow (300W/80W Actual) and I'm wondering if that will help with heat and humidity by not drying things out as much inside?

Humidity matters. But I don't do anything about it myself. The exhaust fan is probably taking away the humidity that watering the plant would normally provide.

If you want to see if you can easily raise and keep the humidity higher, spread out a wet towel in there. Watch your measurements to see if there is any chance you can control the humidity.

How many cfm is your exhaust fan? Humidity, from what I've seen in my garden, doesn't play as big of a role as it's made out to. I've had my RH in the teens in my grow room before, and I haven't seen any differences in growth. I think the problems arise when you have seedlings in a small quantity of starter soil, they might dry out a couple of times a day with lower humidity.

At high humidity levels you'll start running into fungal issues above and below the soil. When your humidity is high the plants require much less water, and they can stand to dry out a bit more between watering.

Humidity is the easiest variable to control if you can control the amount of time it takes for the air to exchange in your tent. If you put a fan rated for a big tent in a small one it's going to drop the humidity, and visa versa. Exhaust fans also affect the temps a lot with HID lighting, but with florescent/led you shouldn't see too much heat building (unless you have absolutely no exhaust).

As far as the negative pressure goes, you should still be fine maintaining it unless you added an intake fan that is more powerful than your exhaust fan. Adding a carbon filter will restrict the flow of air through your exhaust fan, you should see a slight increase in humidity from this. But to answer your question based on my experience, low humidity isn't a big deal, just a pain in the ass. You'll water a lot more frequently at 20% than you will at 60%.

A little info about the physics of water; evaporation slows substantially above 60% RH. Go out and exercise on a mid-western summer day, with a RH around 80%. It feels hot and muggy, sweat soaks your clothes and you get sticky. But in an arid region, even without physical exertion, sweat dries as soon as it comes out, and you have to drink much more water. Our bodies sweat because when water evaporates it absorbs heat (endothermic reaction I think is the term for it) causing the air around the evaporation to cool. That's why 90 degrees F @ 80%RH feels much hotter than the same temp @ 20%RH. The same thing applies to your plants. As water evaporates from the surface of the leafs they cool off. The ambient temp in my grow room is around 72-75 lights on, but a surface thermometer tells me that the canopy is only 68 even though there is a 1000watt MH only a couple of feet away...Evaporation.