Penetrating Damp Saga - Where Next?

[SadLass]

Could anyone offer advice on what to do next with a damp issue?

My other half owns a ground-floor flat that he rented out so he could move in with me. When he lived there, we occasionally smelled damp, but it wasn't after rainfall, there were no visible signs, and it kept disappearing so quickly that we wondered if we were imagining things.

We got a tenant in through a reputable letting agent, and when they moved out, there was a lot of damp. It was showing in all the exterior walls along the rear of the property with a mildew effect.

We got in an independent damp specialist and did everything he recommended in his survey. Basically, the plasterboard was going all the way to the floor, so we redid it with a gap. We put in floor vents. We filled a small exterior hole. We also fixed an issue where the bathroom fan was ending in the cavity. Then we dried everything out with a dehumidifier, painted it, and felt smug.

One year later and a routine inspection of our new tenants - a really lovely couple - has shown that the damp is worse than ever! It's starting about a foot above ground level and then going all the way to the top of the bedroom ceiling. This is despite them using a dehumidifier daily. It's blotchy and seems to be penetrating damp. This is a 1950s property with harling, which appears to be in good condition. The upstairs neighbour says that they have no issues, but they are also pretty combative people. They won't let us see inside their flat and don't like us accessing the back garden, even to inspect the walls.

Outside, the downpipes and guttering appear to be in good condition. We are bamboozled and have no idea what to do next. I'd like to rule out a leak from above, but I'm not sure how to do this without starting legal action against our neighbours, which we don't want to do without proof.

Should we tell our insurers? Has anyone got any good advice? Thank you!

Sounds like there could be a leaking pipe somewhere?

I’d perhaps get a party wall mediator involved to manage relations and access with the neighbour. I really feel you need to thoroughly investigate the property which includes theirs.

You're definitely right about needing access. We're in Scotland, so party wall mediation doesn't exist, but I'm looking into options for general mediation now. Thank you! I didn't realise this was an option.

Our old house it was the drains. Something about them losing water to surrounding ground which was making it to the walls.

Thanks for letting me know. Do you mind me asking how you diagnosed that the drains were the issue? They may well be leaking, but how can you tell? They just disappear into the ground at the back of the house. Do you need to dig?

You can get a drain survey done which will show the condition of the drains. They use cameras to see what's going on without needing to dig up the ground. Costs a few hundred pounds.

Have you checked your insurance? You might have a leak tracing service included?

Thank you! I did not know about these. That's brilliant information.

Actually, no, we've just been paying for everything ourselves. I thought that home insurance never covered damp issues. I'll definitely look into this - the advice is very appreciated.