Chris, Have you got a wet basement or crawlspace? What about a humidifier? Given the uniformity, I'd guess mechanical pressurization i.e. a forced air furnace with return duct leakage coupled with one of the above moisture sources. Let me know if these apply, there a couple other simple tests that can be done.
BTW, double vapor barriers are a no-no; moisture will get trapped between and you'll soon have a fungus farm.