Any pipes or penetrations in the wall above that spot?
Yes. I have a vent pipe for my clothes dryer going through the joist at one end, and the intake/exhaust for the furnace going through in the middle of the wall. I suspect that some of the moisture at one end of the wall is due to the dryer vent, I'm thinking about wrapping the pipe with metal tape to try to seal it up to keep residual moisture from leaking out of the pipe, but I'm not sure that this is the main problem. I had sealed the hole around the vent pipe a few years ago with expanding foam insulation.
Is the spot centered on a joint in the block below or square on one block?
I'll have to take another look at it, but the rimjoist seems to be wettest at each corner of the wall.
The rim joist runs paralell with the floor joists but is tied into the floor joists by short pieces of joist: something like this if you viewed it from the top:
Rim joist -------------------------------------------------------------
I <-24" interval->I I I
floor joist -------------------------------------------------------------
------------------------------------------------------------
Any ice buildup on the eaves this winter. If so, water may have backed up and seeped down behind the siding and interior walls and saturated the plate?
That's an angle that I might have to investigate. Is there a way to determine by inspection if this is the cause? I didn't notice any ice on that side of the roof. The house is a ranch style, with the north end being one of the ends of the house (peak of roof on that side). The very edge of the subfloor where it rests on the joist is damp in a few places.
How did the insulation look?
The insulation looked fine with the exception that it felt damp.