"Nicola O." wrote

The wall seems dry where the fixtures are,
but the grout looks really crappy. It'll come down, and we'll see
what we shall see.

cshenk, the studding is at least doubled in almost every location.
There's like 6 verticals in the back corner (!!) and two sets of nail
holes. We moved in just 2 years ago -- I'm thinking the owners did
for resale exactly what I'm doing-- now sistered in the studs,
retiled, and did a crap job on the grout.

Humm, thats bad news. It's fixable still depending on how handy you are.

Are you comfortable framing a wall? If you are already sistered, you can't
add a 3rd layer.

A few questions on things that are not clear to me then my best guess on
advice.

Q1: This is an outerwall which is also a shower tile enclosure where the
tile goes up pretty close to the ceiling right? The shower spigot comes out
within the tile?

A: if both right, it may be a pressure leveler gone bad behind that time
with a slow leak which caused the grout to go bad. Sealed on outside, not
on inside. This is what you hope for as it's the cheapest one to fix. Rip
all the tile down and turn on both hot and cold at full blast while looking
at the pipes. What you hope to find is a pinhole leak.

Q2: This is an outerwall and you can see the outside wood (clpabord or
whatever) is right against the insulation that hasnt been pulled down yet.

A: this is cheap construction but it was done then. The answer isn't
completely cheap but if you were thinking to reside the house, they add the
water barrier to the outside of the vinyl siding. Older houses that predate
this can get some help from adding it internally but you'll find the wood
siding is nailed to those framing parts so you have to remove them to add a
water barrier between them and the outside. Proper caulking and angled
siding removes that problem as long as the gutters are functional.

Q3: None of the above seem to pertain.

A: you may have a roof problem. A seller will not tell you this and will
cover damage for a fast sell. To discern this, you need a big hose and hit
the wall and roof for a goodlie time with someone else inside. If it's the
roof, you'll see it pouring or dripping in. This may well be along the
edges where the window is (traveling over to the shower) or from above and
just dripping down. Be patient. This check takes a good soaking. What you
are hoping for is if this happens, it's when you hit the window or just
under the roof. Caulk may fix it.

Thats the best I can do other than to mention we get alot of wood boring
bees here so we tend to develop holes along the wooded part of the 'just
under the roof' eves. Also, if already sistered and those are bad, you
really need to reframe tht part but not til the water ingress is identified.
Rule out the rest and it may have been bad internal grout, the the existance
of already sistered studs, indicates otherwise.