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?

+1 to the picture.

Until then... best practice is no vapor barrier.  You want the wall to breath/dry to the interior.  There also shouldn't be any batt insulation directly against the basement wall.

Ideally, you want something like this:

That's how I did a section of mine

1" Polyiso on the walls with a EPs  strip on the bottom taped and sealed. Unfaced bats in the studs.

He rest I plan to find some reclaimed 3" polyiso and use during strips for the drywal

Finally able to load some pics ...

Bottom of wall


Top of wall


There is no sign anywhere of moisture or dampness on the inside of the poly.  I've actually removed a section of drywall in an area I need to repair, and the insulation is nice and yellow, like new.

moisture/mold damage

Even with the cavity in the corner with no evidence of the discoloring? Not every cavity looks like that, it seems to be ones with wiring or piping coming up through drilled holes in the top plate.

I do think that I am going remove the poly from the unfinished walls, as it seems like the consensus is to allow any vapor to dry the inside.

Also, no evidence at all of black discoloration on any of the studs, or on the concrete wall.

Treated studs?

Usually only the sill plate would be.

Get it tested for mold. Moisture might be coming from the wall/cove and then trapped by the sheeting. Inorganics are best for a basement.

I was a marketing manager for a basement company. 

Sill plate is treated, the rest is just regular lumber. I've also had my hygrometer behind the plastic since yesterday evening, and it's showing 51-52% RH.

For testing, are there do-it-yourself kits, or are they junk and I should call someone?

Thanks for the help on this.

The kits in home depot you collect a sample and put it in a petri dish then ship it to the lab and they do an analysis. You get a printout with the type and how hazardous it is

I believe that's called mold son.

This.  Condensation on cool surfaces.  What is the humidity of the basement?
ETA, answered. 70 degrees F indoor air temperature with 50% ish RH will condense at 50 F.


Dehumidify.

Winter it runs 45% constant, dehumidifier hardly ever runs. Summertime I keep it at 45-50%, might get close to 55% during real humid weather. I keep 2 vents open on that side of the basement all year, and the returns are toward the opposite end.

ETA: I pulled a couple batts of insulation off the wall, and the back side does not look like the front, no discoloration at all. Looks new. Nothing on the wall that looks like mold, or that I can even scrape or rub off.

Opening that pathway at the top allows very slow air movement over the face of the insulation.
Conduction is attempting to draw the heat out of the insulation to whatever the outdoor temperature is.
The small amount of convection isn't enough to warm the insulation sufficiently so that it is above the dew point.

Opening it all up is an option, however you may find that the condensation problem just moves further back towards the wall behind the face of the insulation.
In my house I had some mold/mildew between the block basement and fiberglass insulation, with minimal or none on the inside face of the insulation like yours.

I wound up digging my foundation out (different problem) and putting 2" XPS on the outside.  Nice and warm down there this winter.

Nice
An uninsulated basement can raise a heat loss by 10k