Condensation between layers of insulation?

I've got a 1.5" layer of XPS foam against a cold concrete wall with 2x4 framing inside that. I temporarily placed a couple pieces of R12 fiberglass batt in the 2x4 wall. When I removed the batts the next day, there was a considerable amount of 'sweat' on the XPS. No moisture was detected on the XPS where it was left uncovered.
The area is not directly heated and currently sits around 55 degrees. Exterior temperature is closer to 45 degrees.
Any ideas why condensation would occur between the two materials? It's not obvious to me. The plan was to fully insulate the walls with fiberglass and then cover with a plastic vapour barrier...now I'm not so sure that's a good idea.
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You have effectively put a vapor barrier in the middle. Moisture can't pass through the xps and with the batts on top it is a cooler surface than the room.
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In

That sounds reasonable offhand, bee. Also there might be the possibility that any air flow has been stopped or nearly so, removing any way for moisture to seep out so it condenses first.
I wonder if, in that area, if only unfaced insulation or even none, wouldn't be better to do. Concrete, contrary to what a lot of people think, and its appearance, will in fact leak moisture in through it, so stopping that possibility alone can't really be done at any reasonable cost.
HTH,
Twayne`
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Yes, the XPS is essentially a vapour barrier. What I don't understand is why presence of the batt would create a temperature differential large enough to cause condensation (more specificially, wouldn't temperature difference between air and exposed XPS be similar)?
My assumption here is that moisture is coming from the ambient air (which has relatively high humitidy...air exchanger is currently on the fritz). Once covered with a plastic vapour barrier, that ambient moisture will no longer be able to condense inside the wall. Valid thought? FWIW, I've been instructed to cut slits in the XPS to reduce the double vapour barrier effect.
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first question before anyone can answer you is:
are you talking about a concrete BASEMENT wall or an OUTSIDE wall?
Mark
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Basement, approx. 7' below grade.
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