Condensation on poured basement walls...

Just bought a new townhouse in September. The basement area is small because it's not the full footprint of the house. About half of the basement is above grade. My husband has a 120 gallon fish tank and a 40 gallon in this room. The back wall of the basement is insulated with fiberglass insulation (there is plastic on the outside). The insulation is about half way up the wall.
Since it starting getting pretty cold, we are noticing condensation in the back right hand corner of the basement, but nowhere else (we've tried to check behind the insulation and don't see anything). I've been running a dehumidifier to keep the humidity around 40%. We see the condensation forming about half way up the wall in the corner which is above grade. I understand that this happens when a cold surface (the concrete) comes in contact with humid air. I just don't understand why we're only seeing it in a small area and why it's not happening all over the concrete in that corner. The corner isn't insulated...the insulation runs up to that point and then stops.
Is it possible that something's wrong with the concrete there, is this simply the coldest part of the basement? There are large windows in the room that have absolutely no condensation on them. I'm just trying to determine whether this is something I can go back to the builder about, or something we'll have to deal with going forward.
Thanks, Jamie
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it is only a single spot. Any ponding during rainstorms right outside the wet spot? Is there a downspout near there? A had a similar problem in one corner, and one of those currugated downspout extension things cleared it right up. Could also be a flaw in the waterproofing or perimeter drain at that point in the foundation. Try the usual trick of taping a square of plastic wrap over the spot, and monitoring it- that will tell you if the water is coming through the wall or not.
aem sends...
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You know- you could get some useful responses _if_ you could tell a story. Quick, simple, cogent would work for somebody who's not there. Not trying to be cute here, but you should consider the plight of the reader. Like: where is the insulation? Where is the moisture? Fifteen words or less, each, for a clear word-picture. How cold is it (in deg F, please)? What kind of windows? Describe the insulation and inner wall surface, please. The "back right (sic) hand corner" is oriented where on the compass?
Concrete has lousy R-value. Exposure to wind and lack of southern exposure can markedly reduce its temp at an outside surface. It sounds like you've got some insulation on some of the basement wall. Consider going from floor to top of rim joist with as much as can be fit, with some sort of inside covering, like maybe sheetrock for fire-safety.
Assuming there's no leak from the outside, since you're certain it's condensation. J
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