You rent? Tell your landlord.

If there are patches of damp, there may be bridging between the outer and inner wall allowing moisture to wick across,

Possible causes of this are debris in the cavity, cavity wall insulation etc. Often happens below windows for some reason.

Painting Thompson waterseal on the outside wall worked for us, along with having the cavity insulation removed.

Leaking gutters also can cause this, or make it worse if the leak runs down the wall.

All these things are fairly pricey fixes and your landlords responsibility.

Edit Just read you cleaned the gutters out, that's a good start, Also the state of exterior pointing can cause issue.

Often your issue is pretty much the opposite of what you might expect. Older houses were built with an expectation that there would be airflow, and usually with lime plaster walls that are breathable (and lath and plaster ceilings that insulate relatively well - heat and sound), and often without cavity walls (so single skin) etc.. So they act differently from later construction. A lot of the time that plaster ends up either being replaced by gypsum, being skimmed with gypsum, or being wallpapered/painted with materials that aren't breathable, and people block off the various ways you used to have airflow..

When people insulate, especially if they do it relatively cheaply/simply add insulating material to cut air flow, you end up in a situation where you don't lose heat, but you do retain moisture and you end up with condensation. The result is then really commonly wet wall areas (spots if you have bits of patched plaster for example, or corners where external walls are cold), and then mould. It can be more of an issue if you have lots of layered crap on the walls (think multiple layers of wallpaper and then paint...), especially if you also end up with damp between layers...

That said, if you are stripping the wallpaper (and good luck with that if it hasn't been done in a while, I think the record we had was two dozen layers...) take a look at what is behind it. You should be able to spot lime plaster (it'll look more like concrete), if that's what you've got, and its in good condition, then let it dry out before you do anything else. If it does dry out nicely then you are on to a winner and I'd suggest taking a look at the wallpaper you use, some are permeable/breathable, some are not, you'll likely want something more permeable.. If however you can't dry the room out (without using a dehumidifier...) when you have the wallpaper off, you'll probably want to talk to someone about balancing insulation with airflow, if that's even possible.

Although if its a rental, speak to your landlord first..

The wall is getting regularly soaked or water is condensing there.

Check the outside when it's raining to see if water runs across or splashes near that bit of the wall (doesn't have to be exact), and get a humidity sensor, if the room is humid, buy a ~£100 dehumidifier, not one of the tiny protable ones, a suitcase sized unit. It will take a few days running non stop to dry out the room, pulling tens of litres of water out.

The dehumidifier will make the heating more efficient, speed up drying clothes and stop mold as well.

Took about a fortnight to dry out our flat when we moved in and that was all from long term condensation.

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I've got a 2 family house built in 1900. We had this same thing happen in our upstairs apartment the year that we got the house insulated with blown-in insulation. Our tenant told us that his apartment was crying. We went up to look and the walls and windows were dripping. We talked to a couple contractors and they all told us that it's fairly common in older houses when they're "over-insulated", and they all advised us to get a dehumidifier. We got our tenant a dehumidifier and our tenant says he has no more issues with it.

Are you the renter or the landlord? If the former, call the latter. If you’re the latter, there’s already some good starting points posted.

Dehumidifier is your only option

Lots of potentially good answers already, so I will try to keep my (initial) post short: before you can do anything to address the problem you must determine whether you have a leak from outside or have a condensation problem inside. ... I am assuming you have considered and discounted a plumbing leak.

If it's a leak from rainwater the problem will likely be year-round. If it's condensation it will likely be worse in winter .... unless you have air-conditioning which can, under some circumstances, cause condensation issues in the summer.

So solutions- if it's a leak, any plan must start with stopping the water entering the structure from the outside. All the interior wall treatments and dehumidification will be for nothing if the moisture is still entering the wall cavity, as it will just cause mold and rot.

If it's condensation, then running your AC will help, or a dehumidifier if the problem is only in one room. Also move furniture away from exterior walls to allow air to circulate. A fan (rather than a dehumidifier) might solve the problem if it's minor. In the winter an auxiliary heat source, such as an oil-filled radiator placed near the damp spot should also solve condensation dampness. .... When I was a child, I used to have a book case at the foot end of my bed, up against an exterior wall. We found mold had formed on the wall behind the bookcase, due to coldness and poor air circulation. Swapping the bookcase with a small radiator that already heated my room, entirely solved the problem. ... It also helped keep my feet warm! 😀

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