You state that your attic is humid. The attic should be at the same humidity as the outside unless it is getting moisture from the interior (or water getting in from the roof). The attic cannot create humidity. You state the house is "tight" but that is relative, do you have the CFM50 for leakage? I still believe a blower door check used for diagnostics with smoke and probably infrared scanning would be more telling for air leakage. Adding insulation is easy enough but if air is moving though it the r value will be compromised. Knee walls are tricky areas, the floor cavities under them need to be sealed. Turning off the heat does not stop stack effect. Blown cellulose is better than fiberglass IMO. Good luck and let us know what the final fix is.