ᐅ Yellow stains and mold on walls adjacent to the chimney

Hi everyone,

We bought a house last year, and around January we started noticing a yellow stain on a room wall next to the chimney, which kept getting bigger. By the end of winter, it seemed to calm down for a while. Now the issue has come up again, and in the laundry and changing room, mold has even appeared under the wallpaper next to the chimney.

I first removed the wallpaper and took off the plaster, which was extremely porous and partly swollen.

I suspect there might be deterioration of the chimney, and we may need to invest in an appropriate renovation (a liner inside the chimney?). Has anyone had experience with this and can possibly make any conclusions or rule anything out based on the pictures? I guess the chimney sweep would be the best person to consult, right?

Thanks in advance for all the replies.

P.S.: We have a gas heating system, not a condensing boiler. It pushes 170°C (340°F) into the chimney – I had hoped that would be enough to carry all the moisture out :-o


Back of the wallpaper:

Close-up of wallpaper:

Wall with porous plaster:

Wall on the other side of the chimney (no wallpaper, just plaster):

Was the chimney previously used for wood or coal stoves? Old soot deposits are difficult to renovate. In such cases, using a lining or a flue liner is often the only practical solution.

Phew, this thing is 60 years old. I believe it has always been oil-fired. It has a triple-flue, so it might have had a stove connected to it in the past.

It’s probably due to dampness, yes. The same applies to our chimney in the attic. At our place, a plastic pipe was installed for the gas heating system and the roof was renovated. There’s not much you can really do, it tends to come through everywhere because it’s in the masonry. In our case, even after several repaintings, it turned yellow again. You could try replastering or cladding the chimney after consulting with the chimney sweep. Since it only affects our bedroom, we decided to leave it as it is.

This spring, I dealt with similar stains due to work reasons. The cause was a chimney flue for an open fireplace that was no longer in use. There was no cover on the chimney. When rainwater collects inside long enough and nothing evaporates due to the chimney draft, it becomes damp and looks like that. Some chimney owners had small roofs installed on their chimneys, while others placed newspapers inside at the bottom, which they regularly replaced. Both methods helped.

Would it be possible to dry-heat the chimney, install a small roof on top, apply a barrier layer to the still intact plaster (damaged plaster must of course be removed), and then it would work again?

There are sealant coatings that block this accordingly (if the cause is resolved). New plaster needs to be applied anyway. Otherwise, 170 degrees Celsius (338 degrees Fahrenheit) is obviously not sufficient.