UKC

Anyone with any damp-proofing expertise on here want to offer an opinion on the cause of this? This is an external wall (house is terraced but this corner juts out). There are a number of possible candidates...

https://imgur.com/gallery/4ETTWAm

1. There is a duct where the wires come through, could it be leaking water in? Seems a long way for it to spread and also seems to have spread upwards.
2. There are various plants in contact with the wall, could they be making the wall damp?
3. There are downpipes nearby, could it be rising damp from the drainage?
4. Could this be a 'cold spot' which was missed when they put in the cavity insulation and this is just condensation?

Answers much appreciated as I'd like to get this sorted quickly. Will be checking out all options myself tonight before getting professional help if needed.

I'd say a combination of cold and poor air circulation, assuming that cupboard is usually shoved right into the corner, which itself is looking a bit ropey on the back!

What is the construction of the wall.

I would say keep, the cupboard thing away from the wall and try a dehumidifier, maybe you could borrow one.
Also when cooking and having showers, have the extractor fan on, or open the windows. Cooking and washing and breathing chuck out loads of moisture and houses are very well sealed nowadays so that moisture will go to the coldest spot with poor air circulation.

One thing I would suggest is not to get a "damp specialist" in, as all they are is "damp proofing salesmen" . Rather like Dr. Leech from blackadder, all you will get told is to remedy you need to drill and inject a damp proof course, hack off all internal plaster, replace with waterproof plaster to 1 metre. 

Is there a picture of the outside - and how high is the ground levels outside compared to inside?

Bizarrely if a standard brick wall it could be caused by the cavity wall insulation. The outer layer is porous, moisture is expected to penetrate the bricks and mortar. The ventilation on the outside of the wall removes it. Many botched retro insulation jobs, block vents, reduce air flow and in effect turn it into a solid wall.

Damp course? As in from the ground up, how far above the ground is it, have borders with soil and plants been built up above it? 

My guess is this is caused, as other have said, by poor circulation of air due to the piece of furniture shown in the photos.

Is the room itself a source of damp air - a kitchen/diner, close to kitchen or a bathroom, or is the corner on a cold area of the house? My late Mum in law's 3rd floor flat had a similar stain in a room on the NE corner, but not in a similarly exposed corner on the SW of the flat.

Agree about moving the furniture away from the wall, also wash the wall with mould remover. A dehumidifier is worth a try but just getting better air circulation  - opening a window or using a fan (appreciate not the best time of the year to do this) could do the trick.

A dehumidifier will cost a bit but possibly less than getting someone in to look at it.

Dave

Thanks all. The book case (which yes has also suffered as have the books) normally has about 50mm clearance and that wall is probably colder than some others. No sources of extra moisture in the area though (it's the living room). There is another book case on a similar wall the other side of the window which doesn't have the issue.

It could just be condensation, but seems strange it's so focused in one spot.

Clear everything away from the outside wall, and ensure that there's nothing out there that can get wet and breach the damp proof course.  Check any overhead drainage (gutters, downpipes etc) that may be leaking onto the wall and penetrating the brickwork.  Check the mortar on the external bricks; is it broken? Does it allow water to collect and sit on it when it rains/gets wet? If so, that could also be responsible.

A corner like this is the classic place for moud to grow on the inside as there is a greater external surface exposed to the elements, whilst the internal surface area is low compared to the volume of wall behind it. It will be almost certain that the source of moisture will be from the outside, so look there first (you've mentioned some possibilities):

1. Check the guttering for any leaks.

2. Check the downpipe that you mentioned - poor joints will let water track down the pipe, across brackets etc.

3. Check the gully into which the downpipe empties. Ensure it's empty and freeflowing. Also make sure the downpipe channels the rainwater into the gully properly and it's not just splashing around and soaking the surrounding ground.

4. You've mentioned plants, - cut them back from the wall, maybe cut them down if they are big enough to be blocking the wind from helping the wall to dry (although if they are that big, they may well be protecting the wall rather than adding moisture). Also check the roots, make sure they're not damging the wall.

5. Look at the ground level: it may have been added to over time, so may need reducing. Also check that it's allowing water to fall away from the house rather than towards. 

6. If you have plants outside, is all the ground along the wall drainable? Any hard surfaces (paving, tarmac etc) will trap moisture which can then spread into the wall. Rain will also bounce off hard paving and contribute to damp walls

7. What is the wall construction? Dense, low-porosity materials will hold the water once they are wet. Cement renders and similar, often added to try and block damp, actually make things worse. 

Can you post a photo of the outside, please?

Go through all the above points and I'll bet you'll improve things (I appreciate point 7 is not a quick fix). Also do what others have suggested internally (keeping condensation down, heating, ventilation etc).

Finally, and most importantly, do not get any so-called 'professional damp expert' in. They will try and blind you with science (sticking a moisture-meter on the wall - you can see it's damp FFS) and will play on your fears to try and charge a fortune for 'remedies' that are dubious to say the least.

The remedy is often fairly straightforward and can easily be done yourself, just try and take a holistic view, don't panic. Let us know how you get on.

Make sure they are actually draining and not blocked otherwise your guttering might be overflowing and pouring down the wall when it rains (we had this).

Black mould = condensation damp. Black mould likes to live on pure water, penetrating or rising damp isn't pure water (dissolved salts etc.) and black mould won't grow on it.

It's most likely to be due to stagnant air in the coldest part of the room, however you need to consider why it's in that part of the room and not elsewhere. You've already given good reasons for the coldness - projecting corner, duct & wires forming a 'cold bridge', plants shading the outside wall (possibly), plants making the outside leaf of the cavity relatively damp (almost certainly), difference in insulation (possibly).

As stated upthread, cavity walls combat penetrating damp by allowing any water penetration* to run down inside the cavity instead of penetrating the inner leaf. If cavity insulation is fitted which touches both sides of the cavity e.g. foam then there is the risk of a 'damp bridge' which will make the inner leaf cold even if it doesn't penetrate.  I have black mould in the alcoves of my late Victorian house as the walls are only about 8" thick compared to the 2'6" everywhere else. One area is particularly bad due to penetrating/'rising' damp in a chimney breast making it colder than elsewhere.

You could get a thermal imaging survey done or simply find a mate with one of those CAT phones with a thermal camera on it. Using a moisture meter may be of some use if the readings change with rainfall, but be aware of their limitations when used in non-wood substrates.

It's probably not a good idea to have plants on the outer wall, especially ivy. Although they may not necessarily destroy the wall with their roots the wind pulling at them will.

How to fix the problem - raise the room temp and thus the wall temp above the dew point so there's no condensation, remove the source of water vapour, use anti-mould paint/spray?   Fruitbat has some good advice.

*If the outer leaf is bricks with cement mortar then you WILL get water penetration from driven rain. The cement mortar in the vertical joints will have shrunk and detached from one or both bricks either side.

Thanks for lots of very useful replies. Based on your advice it's definitely condensation. I also had something of an epiphany when I realised that the reason it's suddenly got so bad is that we've had to dry a whole load of laundry indoors recently - time to look at tumble driers and dehumidifiers!

If you've got an electric fan, put it so it's blowing air round the back of the cupboard and corner.
Possibly work wonders!

It's worth taking clean washing to the laundrette just for drying to avoid mould until you get another solution.

If you do have to dry laundry indoors then you probably should turn the heating up and open a window.  Not very energy efficient but better than mould.

As others already said get some anti mould stuff and clean the wall.  

I'd probably ditch the back of the cabinet and get some replacement hardboard from a diy place.  It's easy enough to cut.