On Wed, 25 Jul 2007 12:27:06 -0700, Abe wrote:

wrote:
On some interior walls in my house, I'm noticing dark patches that
seem to be appearing between studs. SNIP In the largest spot (about 3'
high), I can clearly make out where two studs straddle the dark area,
as if whatever is causing the darkening is trapped between the two
studs. What could be causing this discoloration?

Moisture would be the number one likely culprit...

Inadequate or nonexistent priming could be another.
I have to go with priming - over a lousy insulation job. As air moves
back and forth through the wall, bigger dirt particles get trapped on
the wall's surface. Only you will know if you can/should get more
insulation and a proper vapor barrier into that wall. In the interim,
I would scrub the wall and then prime the entire wall with one of the
commercial products like KILZ which advertizes it will cover anything.
Years ago (maybe still do) Glidden sold a vapor barrier paint for such
problems. Other mfgrs may.
You could get what you use tinted to match the current color if you
are enamored of it.
HTH