What is this patch on the wall?

[Gingerninja1981]

Lived in the house 6yrs. It's a 1930s semi. This wall is the inner side of an external wall. Dining room one side, garden on the other.

I've painted this wall a couple of times since I moved in, so it's had around 4 or 5 coats of paint.

I recently re decorated again and when I was doing this wall, the paint was "lifted" on a small maybe inch high very thin line of the wall.

I got a wallpaper scraper and it resulted in all of the paint coming off in pieces underneath the windowsill, almost like wallpaper would.

I sanded down the edges where the paint hasn't come off higher up, then used polyfiller to smooth it out (there is no polyfiller on this darker patch).

I then did a pva mist and coated the wall where it was bare plaster.

I later painted it. Once dry. This darker patch appeared. I used an electric heater to try and dry it out. Thinking the mist coat hadn't fully dried.

I then did a second coat around 3 weeka ago. It's been fine, no darker patch, until it's rained heavily today when it's reappeared.

There aren't any leaks on guttering, can't see any big cracks or gaps in the brickwork, nor any gaps around the windowsill.

It's not doing it on the higher part (between windowsill and the dark patch) where the old paint didn't come off.

Any ideas was to what it is? Thanks in advance and sorry it's a long post, but I'm trying to get as much info included in the hope it rings a bell with someone.

Could rainwater be getting in that airbrick? I'm pretty sure they're meant to be higher up. Looks like the concrete outside has been added since the house was built and that's made the ground level too high so water can get in the holes.

It’s damp. Is that drain blocked? In any case, that air rock will let water flow into the cavity and hence the damp on the inside wall. You need to lower the ground level a bit by the wall.
Ideally if that drain is in use, you’d lower that also.
If you leave the hose running in the area (to mimic a lot of rain), what happens?

Air rock? Air brick!

Thank you for responsive with your thoughts.

It's not a drain, it's wooden frame for a trellis and then slate chipping at the bottom. Does look like a drain having looked back at the image.

So, air brick is likely culprit. Odd that it's not causing issue on the wall elsewhere or before now.

Though, perhaps it's the new paint colour and lesser layers of paint that have made it now visible.

Does seem the likely culprit, but with it only just appearing to be happening I was dubious.

Pneumatic drill rental Google coming up 😆

I made a spelling error in my response to you too. So it's 1 for 1.

Has there been repair work to the brickwork outside? At all, ever? It looks like newer cement on part of it.

The damp proof membrane (which will sit above the airbrick) might have been damaged - especially as there's a cable nailed into the cement row where it sits.

The cementing doesn't look properly finished either.

I reckon the path/patio outside has been laid too high and has bridged the damp proof course.

Yes the pointing was redone as there were gaps in the brickwork

Yeah I need to look at the cable too. There's are so many cables on the house, that's on the to do list.

Having had another look at the inside/outside pics, I really do think that the cabling is nailed in the wrong place and has punctured the damp proof membrane so if you do move it I think the problem will still happen until the membrane is repaired.

If it was the pointing, the damp would be more evenly spread over the wall rather than looking like it's soaking up from the bottom.

That cable looks as if it's feeding the router(?) - the bigger white box on the right under the window.

I'm going to get out and take a proper close up look at it all and check all around the cable and then clean around the bottom of the air brick and tape the bottom row of holes up for a few sdaysand see if it continues whilst it rains.

Try and Isolate the issue.

Thanks for your advice.