What is causing this damp? hydroscopic salts or something else?

1 Mar 2011
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West Glamorgan
United Kingdom

What is the cause of this on the wall of my daughters bedroom, its just showing in 2 places, where the paint has cracked off, and now its looking a bit black underneath. Also after periods of heavy rainfall it smells damp in the bedroom.

It has been like this for months and could it be hydroscopic salts in the chimney breast which is to the right of the damp patches on the picture.

In the last few weeks we have had the chimney stack taken down to roof level at the eaves and the chimney breast is venting into the loft. We had the stack removed as it was not being used and wanted to eliminate this as a cause of water ingress into the chimney.

On the exterior wall it is clad in clay tile, timber frame with rockwool insulation (1960's property, in the living room below in the fireplace is a standalone electric fire in the fireplace. When taking the electric fire out the chimney comes down into which looks like a baxi cast iron part. The wall in the living room where the fireplace is recently wall papered but we never have had any damp patches here. However sometimes the room can smell of wallpaper paste?

What is best to treat this wall with to solve the problem? Are there any anti damp paints I could use, as I don't really want to hack all the plaster off (actually this wall might be boarded over then plastered)
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The shape of the damp stain is indicative of moisture penetrating from the rear of the wall, and not being absorbed from the air in the room.

Thompson's won't stop this. You'll need a different product to coat the wall to form a barrier, and then replaster a wider area.

But if it's coming from the flue, then the venting is inadequate, and the damp could just reoccur somewhere else. A disused flue should be vented from the bottom in the lounge and at the top to external - not in a loft as that does not draw air out and can cause warm air to enter the flue and condense.

Penetration from external is possible, and should be checked, but if this has occurred following work on the flue, then that's the most likely cause.
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It was like this before the chimney was done so it must be coming from the outside? But how would you know? I have attached a pic of the exterior, the damp patch is to the right of the chimney breast where it meets the tiles
This is another pic of the exterior

Also you mentioned not thompsons water seal, what product should I be looking at ?
There are two possible causes:

There's no signs of abutment flashing soakers and stepped flashing where the tile meets the c/breast cheek?

Or it could be that its damp and salts (liquidised soot) penetrating from the chimney flues.

Have the flues swept.

You presumably have two flues running up the c/breast so two air bricks could be inserted a few courses below the eaves.

Pics of the whole chimney breast in the bed room, and the floor below, and in the loft would help.

After having the flues swept, & the air bricks inserted, leave well alone for a month or two and see how you get on.

Its typical after any damp or salt penetration (esp where the plaster is bubbling) for the plaster to be removed back to brick, and rendered in sand and lime.

Never board over a damp wall.

My view is that using water sealers can possibly cause further difficulties for little benefit.
Hi thanks for this

What do you mean by abutment flashing soakers? And stepped flashing ?

Not sure if I have 2 flues I think it's just one? Can you still have it swept if there is no chimney stack any more?

Where the damp patches are you couldn't take back plaster to brick as this part of the wall is timber frame no brick

So fit air bricks in the exterior chimney breast just below the eaves and at the bottom, this is there at the moment which was used for emptying the ashes from the fire, so could this be made into an air brick as well? And would this provide adequate ventilation for the chimney ?
If it smells musty after heavy rain, then the likelihood is that the gutters are spilling over and finding a way down the chimney.
There is a lot of algae on that stack, which may indicate excessive water soaking it from the roof level. the roof covering or that flashing under the fascia might need looking at, to confirm that its not pouring water down the brickwork.

If there have been no previous damp issues, then it can be assumed that the tile hanging is not causing a problem.

If the damp started after the chimney work, then that's a good indication of the potential cause - either related to the venting of the flue, or the work they did on the roof.
It was like this before the chimney was removed so could it be the tiles on the wall ?

If so what would I need to look for ?

Don't think it's the guttering overflowing either as it's quite new and there is no visible signs of it overflowing during heavy rainfall
I only use water vapour permeable exterior treatments. They keep wall dry without trapping water. Fluoropolymer type are best
My feeling is that when the rain comes from a certain direction, it gets " caught" in the corner between the tiles and the stack. So the side of the stack gets flooded and the water runs in past the edge of the tiles. The tiles should have slaters felt behind them with a waterproof seal to the side of the stack, so any water that gets in stays on the outside of the felt. This is where I feel your problem is:- No seal, bad seal, tear in felt. . . Real PITA to fix. You could prove this (perhaps?) by pushing some tissue paper up under the tiles right in the corner. Not conclusive but if it gets wet, then there is water coming down the corner the wrong side of the tiles.

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