?Damp? marks where the walls meet the ceiling

[Avocet]

Can anyone help with this, please? The house was only built in 1995. Concrete blocks, with a cavity. The inner leaf blocks have patches of foam sprayed on each block for insulation. The outer leaf is pebbledashed. The interior walls are just plasterboard stuck to the concrete blocks with blobs of adhesive, skimmed with plaster, and painted with magnolia emulsion.

The house is very dry and centrally heated.

Over about the last 10 years, maybe, I've noticed spots appearing on the walls and ceilings. They're always on external walls, never internal, and they're worse upstairs than downstairs. They are always in the corners, and the area where the ceiling meets the wall. I've attached a few photos of the worst room, which hasn't been decorated since the house was built, almost 30 years ago.

Around 10 years ago, we had more insulation put in the loft. I don't know if it's related, but these problems only seem to have started since then.

Could anyone give me any ideas as to what is happening and how to treat it, please?

 

[RandomGrinch]

Have you noticed any condensation on the roofing membrane/felt, in this cooler weather?
Is the extra insulation blocking eaves ventilation, is it sodden at the edges?

 

[Wayners]

It's common especially in bungalows because there is a joist near the wall about 50mm away. The plasterboard gets cold along edge because there is no insulation in that gap as it was missed.
Cold plasterboard creates condensation on surface.
Another problem is the plasterboard is against the cold wall and again this causes condensation along the wall and ceiling line.

Dehumidifier run for a few hours every evening and used to dry washing with eliminate moisture in the house and help.
It's modern living with warm moist air hitting cold surface

 

[Avocet]

Thanks both, Randomgrinch, I'll go up in the loft and check those things. (Been away for the weekend). I'll report back.

Wayners, thanks for that. The house is notmally pretty dry. We have an old piano in the dining room, and I have a hygrometer in there, which is saying 42% at present. I might try taking it into the upstairs rooms to see what it says.