Yellow cracks- should I worry?

[DustyDoorframes]

Argh, help hive mind! We are in a maisonette (very early post-war brick built), and my DS’s bedroom wall backs onto the unheated, open stairwell to the flats upstairs. It’s only internal thickness so therefore a really cold wall, much colder than the actual external ones, and makes the room very cold. When we moved in there was condensation and peeling wallpaper on that wall, so we’ve been careful to ventilate well and not put anything too solid against it. We are ex-council, so we’d be waiting til kingdoms come before the council take action on this! So, on Monday we’ve got our own builder coming to insulate the wall internally with celotex boards and skim it.
Having taken down the stack of books on the end of DS’s bed, these yellow cracks and patches have emerged behind- and now that the posters are down too, they’ve revealed more hairline yellow cracks too. I’m having a little panic because of the yellowness and cracks- I was expecting condensation, but is this a sign of penetrating damp?? Are we going to encase the problem and make it worse if we insulate over it??? Heeeeeelp!!!!

I'm not sure - but your builder should be. It seems like plaster cracking due to expansion and contraction as it dries. You're probably getting condensation on that cold wall that's affecting the plaster. The insulation should stop that. As I say, check with your builder. I'd be very surprised if it was penetrating damp as it backs onto an internal stairwell, so where would the damp be coming from?

Thank you @FenghuangHoyan that’s very reassuring. I’m a bit stressed out in general at the moment so I’ve lost common sense! Also it’s a builder I don’t know, so I’m slightly worried he’ll turn out to be a cowboy…

Sometimes yellow marks are caused by water coming through the wall after a leak, but yours look more like old wallpaper paste or distemper. See if they will sponge off with warm water. If not, try boiling water (use tongs to hold the cloth).

thank you @PigletJohn ! It wipes off easily with a warm cloth- some bits even wipe away with a dry finger! It doesn’t feel powdery or gritty at all when doing that, just like wiping a smudge of felt tip
off a table or some thing. Here’s a better photo too.

I’ve also just realised- we had the whole place painted when we moved in, and the lining paper was peeling off on this wall. I’d not thought about it, but the decorators must have taken it off and didn’t replace it- so that would explain wallpaper paste coming through on this wall and no others.