What do you have for siding? Do you know what you have underneath the siding for sheathing? Do you have a humidifier on your HVAC system? (Heating, Ventilation, and A/C system) Do you get any standing water on the plastic on the crawlspace dirt? (It is dirt?) Do you have a 1-story home? (Is it a rancher or what?)
Assuming you don't have, say, tar paper under your siding, which was at one time (and sometimes still is) used as a vapor barrier, and assuming you have a 1-story home, and assuming you have little or no standing water on your crawlspace plastic, and assuming your vents in the crawlspace are NOT obstructed, and assuming you don't have a humidifier on the HVAC system, (whew), I'd venture to guess the source of the problem might lie with the fact that the house is tooooo tight.
If you have plastic in the attic that's being used as a vapor barrier, that could be keeping humidity in the house. Also, things like taking a long, hot shower w/o venting the bathroom, or, doing a lot of cooking can keep moisture in the house. There is an expert or 2 that visit this Forum, and maybe they have more insight. But to me, in short, the house is unusually high.
This sounds like a strange question but other than 'feeling' like it's humid in the house, do you have any signs of mold/mildew??? If so, where is it???