1930s semi damp on gable cavity wall - What's this in the cavity?

[drpepe]

Hip to gable conversion about 5 years ago, and now stripping back wallpaper on the old part of the gable wall to find blown plaster and damp just to the side of the 1st floor landing window (you can see area of skim that came off quite easily). Also found several spots of damp and peeling (new)paint in a couple other places on that wall at 1st floor height.

South facing wall. Original wall has a cavity, with the new gable built of timber on top. The render outside is not great, and has obviously had cracks repaired in the past. The conversion from hip to gable may be subjecting the wall to more rain than before. However given the cavity I am not sure how moisture is getting through at all.

Going in the cavity from the top left of the window with an endoscope, I can see one or 2 small bridges of mortar,

lots of retrofitted cavity ties, and this big black shiny thing across the cavity just above the window, spanning the cavity and about 15cm or so wider than the window on either side (the pic is looking up at the part projecting past the window's wooden beam from below). Looks like some sort of damp proofing but could it be channeling moisture across the cavity?

 

[noseall]

Going in the cavity with an endoscope I can see one or 2 small bridges of mortar, lots of retrofitted cavity ties, and this big black shiny thing across the cavity, projecting about cm or so wider than the window on either side (wider than the wooden beam). Looks like some sort of damp proofing but could it be channeling moisture across the cavity?

Could also be cold spot thermal bridging, causing localised damp spots. The stair wall being one of the most awkward to insulate internally, because of encroachment onto the stair stringer.