Okay, I'm back home, I came up with a list of questions on my 3-hour drive that may have an effect on your total room environment. You are essentially in a hole in the ground and these may have a greater impact on you than a ground level room.
Besides the 100sqft grow room is the rest of the basement dirt floor? how big is the common area of the basement?
Do you have a sump pump and does it run? how much?
Do you have an active cistern?
Do you have gutters and downspouts? A 1000sqft roof will dump approximately 600 gallons of water per inch of rain down your basement walls.
Do you have 18" thick flagstone walls? have drain tiles?
What are the average outside daytime and nighttime temperatures and humidity levels?
It sounds like you are venting the grow room directly outside? are you also bringing in the same amount of fresh makeup air back in?
Here are some of the things I would check or experiment with;
First get a couple of feet of clear tubing and some food dye and make a water manometer, once you form the U-shape and mounted to the wall push one end into your grow room and tightly seal, leave the other end open to the common basement. With all fans off and no type of air movement mark the water column (they should be of equal hight). Turn on the exhaust fan and measure the column again, if it went up 2" then you would have a negative 4"wc in the room. I would put an adjustable heat register in the bottom grow wall and open it until the water level is equal, optionally I would either slow down and/or install a bypass damper on your exhaust system to redirect some/most of it back into the common area of the basement.
Your 100sqft grow room is about 800cuft, as an example, a 50 cfm fan will suck it clear out in 16 minutes. Do you need that much?
I would exhaust the grow room with a smaller vent fan into the common area of the basement and let that 400 tons of cold basement wall be a heatsink then if that won't keep-up then vent the common area to the outside, same with the fresh air intake.
The reason being is that a larger area is less prone to micro changes in the system, also if you aggressively power vent the small room to the outside then any changes you make to the inside environment be it humidity or otherwise will be negated and will only be throwing good electric money out the window.
Moving the dehumidifier into the common area may also be the better choice, let the small grow room exhaust fan create a slight negative pressure allowing the cooler drier common air to be drawn back through the heat register near the floor. Using strategic bypass dampers and blend doors should also cut back on the need for heat in the winter too.
Now admittedly I am not an expert with these systems as a whole as my expertise was PLC programming and instrumentation but got to see the entire workings while troubleshooting many of the cause and effect scenarios I outlined above - it seems everybody blames the programmer until it can be proved otherwise, sure am glad I retired to a 15 hour a week parts delivery driver. only 100 weeks left until full retirement.
