worried wrote:Yes, it was previously stained/painted.
There is a difference between being "stained" and being "painted".
Most exterior stains soak into the wood and do not seal the wood, and thus do not offer the same protection for the wood against weathering as paint does. Likewise, because the stain soaks in and does not lay on the surface, a certain amount of condensation would be soaked in too and not be seen.
Paint actually lays on the surface of the wood and does not soak in the way stains do, which is why paints offer greater protection for the wood against weathering, and because the paint lays on the surface of the wood, it creates a smoother surface for moisture to bead up on and keeps that moisture from soaking in as it may have had the wood only been stained.
The condensation occurs only on humid/foggy mornings - dew all around. There doesn't appear to be any more condensation on the outside walls than the windows.
You probably see the condensation now as it is no longer soaking into the wood, the moisture is laying on the surface.
Since it wasn't noticeable until the outside walls were painted, is it more likely that the moisture is all from the outside, rather than seeping to the outside surface from within the house?
Probable and likely, yes.
If the fresh paint builds a better vapor barrier, wouldn't this PREVENT more moisture from the interior appearing on the outside walls?
I would use the word "prevent" as the paint is not a vapor "barrier", only a vapor "retarder" which retards the flow of water vapor.
The paint serves to reduce and slow the transmission of water vapor both from the exterior into the wood, and from the interior through the wood to the exterior. If enough water vapor is being transmitted from the interior through the wall and into the wood without being allowed to ventilate out, you will eventually see blisters forming under the paint, and when you poke a hole in the blisters you will find moisture and even water - only time will tell if that happens.
Here is a question: When the house was painted, the painter *did not* caulk along the lower edges of the cedar siding where one board laps over the board below, did they? Hopefully not, as those need to be left open to allow the moisture and water vapor from inside to be ventilated out to the exterior.