Damp patch on the chimney wall

Question

I have a problem with condensation on the wall between my living room and the chimney and I would be grateful for suggestions as to how to tackle this.

Some background: when I bought my house the fireplaces had been removed and the chimneys capped. The chimneys have a vent on the outside wall, and as far as I can tell the chimneys are dry inside.

This was fine by me, but after a few years in the house I redecorated and removed the several layers of old wallpaper on the living room walls and just painted the walls - no wallpaper. Ever since then, if the weather turns warm and humid, especially after a cold spell (it rarely happens in summer), I get condensation on the wall where the chimney flue runs. This is really quite bad and produces a very noticable damp patch.

I'm pretty certain this is just condensation. I think the inside of the chimney gets cold because it's vented to the outside air, and because the wall between my room and the flue is only one brick thick the inside wall gets cold too. Add damp air and you get condensation on the cold bit of the wall.

I believe it's condensation because it doesn't happen when it rains or at all in the summer, only when the weather turns humid after it has been cold. Living as I do in the North-West of the UK that basically means any time in winter when a warm weather front blows in from the Atlantic! I suspect that it didn't happen before I redecorated because the several layers of old wallpaper were acting as an insulating layer so the inside surface didn't get cold enough for condensation to form.

At the moment I deal with it by just turning up the heating, which dries the wall pretty quickly. However it would be nice to have a proper solution but I have no idea what the solution would be. Maybe I should put some thick wallpaper on the wall to insulate it, as it was before I redecorated? Or maybe block off the outside vents and put vents on the inside wall so the air in the chimney doesn't get cold? I did wonder if the entire chimney could just be filled up with the stuff they use for cavity wall insulation, though I suspect that's a silly idea :-)

Response to comment:

Here is my attempt to illustrate what is happening:

The damp forms inside the house on the wall between the inside of the house and the inside of the chimney.

Answer 1

...This is one school of thought: seal the chimney to prevent air flow. By preventing air flow, you also prevent the flow of moisture the air carries with it, and the formation of condensation on the colder, interior face of the flue. FYI, the insulation you push up into the flue works simply because it blocks the flow of air; insulation 'warms' nothing. What it does is slow heat transfer from warmer to cooler areas.

The second school of thought is to increase air flow. The theory is that, with constant air flow, moisture that settles on the inner surfaces of the flue will be evaporated.

For this system to work, the flue has to pull in dryer, ambient air from a lower floor; this requires creating an opening below the damp area. This is the preferred method in Britain, for example.

Incidently, houses built in the US Northeast throughout the 1800s had just such an air circulation system built into their exterior walls. In old houses, you have a cavity between the brick and the plaster walls and that cavity extends from the basement to the attic. Air in the attic is heated and exits, creating a partial vacuum which draws air in the cavity upwards. As the air is drawn upwards through the wall, it is heated, which increases its capacity to absorb moisture. This wicking system removes moisture from the inside surface of the brickwork and prevents moisture setting into the lathe.

Answer 2

If you can wipe moisture off the wall with your fingers, then you can safely say that the moisture is condensate. If not, then the moisture is coming from the other side of the wall, but make sure that the wall paint isn't just absorbing the moisture. We have this problem with bathroom walls in winter: Steamy shower vapor condenses on cold walls where it beads up, causing rivulets to run down the wall. This condensate was serious enough to bubble up the paint until we repainted using exterior-grade paint.

You can solve the condensate problem by, as others have said, sealing off the vent and maybe also insulating the top of the (metal?) chimney flue. You can also add insulation behind the problem wall, within the chimney. Your chimney is a large thermal mass that doesn't respond to temperature changes as easily as your interior rooms: it's cold when the house is warm, and vice versa. Because the problem is a "cold chimney/warm house", you may also want to paint the chimney dark so that sunlight will at least play some role in keeping the interior chimney space warm enough to prevent a cold interior wall....unless the chimney faces north.

Answer 3

If the chimney is no longer in use, but even if there is the slightest chance that years from now you or your children's children wanted to make it functional again, you should reconsider filling the stack with insulating material. It's possibly the easiest fix and maybe the least costly, but not the only way. Similar to what you noted about the wallpaper, installing rigid foam insulation covered by drywall would work. The ideal fix would be to close off the vent in the chimney to stop air movement.

Answer 4

I agree with ojait. But, I might try simple first. Your idea of blocking the vent should be the first attempt at a remedy & done just for winter. Crawlspace vents have this ability to just be flipped closed & it's a common practice for crawlspace owners to shut the vents in the winter because of your same issue. The chimney would then be heated by the house during the winter.

Otherwise, I'd get rid of the chimney entirely. It's serving no purpose, has maybe been the cause of settlement cracks & other issues, is creating a possible future structural problem & its waterproofing at the roof will eventually fail to then cause further & much bigger problems.

If a fireplace is desired in the future by anyone, then the Direct Vent Fireplace insert models can be installed through any & every type of wall & don't allow the home's heating or cooling to be dumped outside.