I have just completed a basement finishing project. I installed 3 registers for heat/AC. I did not plan to put in air returns. We have a radon problem and I did not want to spread the basement air throughout the house. I did the full remediation for the radon and since completing the basement, have had it tested and it is down to .04 now, no different than the average outdoor reading.
But now, I notice more humidy in the basement. It is betweem 55 & 60%. The laminated floor mfg recommends to keep the humidity below 50%. My thought is to add a couple of air returns to pull this air out and recirculate it. Is this viable? The registers are 16" above the floor. Should I put the air returns in the ceilings or near the top of the walls? And how many should I install? The three finished rooms are all open, common areas with no complete seperation between them. The 3 year old HVAC system is gas forced air and it and the gas water heater have their intake and exhaust ported outside the house. They also reside in a utility room that is completely shut off from the finished area of the basement.
If you move that air and the A/C is not running then it won't get dehumidified and will overall could still be too humid down there. I use a portable de-humidifier for my basement and let it dump into the floor drain. You can install an expensive bypass ducted dehumidifier but I doubt you really need that.