We've got a storage room in our finished basement, that does not have drywall up. The 2 exterior walls are the concrete wall, then fiberglass batt insulation, then a poly vapor barrier. The house is about 20 years old, and there has never been any issue with water in the basement or high humidity, which I keep to about 50% RH.
The batt insulation is dark along the bottom and along the vertical framing joists. I'm pretty sure it has to be from air movement, and not any type of mold. But that baffles me because the top plate of the framed wall is exposed and against the bottom of the floor joists. So if there was air current, it would be going into the bottom behind the vapor barrier, up the framed wall, and around the top plate. All within the basement and not going up into an upper wall.
The rim joists are insulated with spray foam and an insulation batt layered in against that. The vapor barrier is not sealed along the bottom or top, just stapled. Can an air current get created like this?