Paintable solution for Hydroscopic salt damp at bottom of chimney breast?

[Red2k]

Hi All,

We have some damp at the bottom of a chimney breast which seems to be due to hydroscopic salts. The wall is plaster (pink) over brick and covered with lining paper and emulsion.

I have been trying to dry out the damp patch with a dehumidifier, fans, hair dryer etc for a week and while it dries out a bit visibly during the day, it comes back overnight. The plaster itself is solid and as the damp is not very widespread I want to try and find the least disruptive solution. Options in order of ease -

1) Paint over the current lining paper with a damp sealing paint and emulsion over - Preferred option - Any suggestions for this?
2) Cover with Wallrock Dampstop lining before lining the wall again and painting with emulsion
3) Hack off the plaster, seal the masonry, replaster and decorate

Thanks

 

[sircerebus666]

You need to fix the cause of the damp first, your solutions merely cover the damp

 

[JohnD]

I have been trying to dry out the damp patch with a dehumidifier, fans, hair dryer etc for a week and while it dries out a bit visibly during the day, it comes back overnight.

so the source of water might be a water leak into the solid floor.

Is the whole room solid floor, or just the hearth?

Where is this room in relation to the kitchen, and the route of the waterpipe and drains?

Stand back and take a wider pic of the whole wall please.

Looking at the height of your wet patch I wonder if there is a pipe or sink above, or rainwater trickling down inside the chimney. Water from inside an old well-used chimney usually leaves yellow or brown stains from the soot.

 

[mrrusty]

It'll probably be because the bricks are loaded with salts from years of fires. I have the same problem. In my case 100 years of coal fires plus subsequently a few years of an open chimney letting the rain down,have left the bricks around the fireplace loaded with hygorscopic salts. I very much it is damp as such, it's just the salts sucking the natural humidity out the air. Personally, I think the only remedy is to chop out the old brick and replace with concrete blocks and replaster, and one day I'll do that. For now, I just live with it.

 

[JohnD]

Why would hygroscopic salts be concentrated down one corner of the chimneybreast, furthest from the flue, and below the point at which smoke enters?

What is behind that cupboard door, next to the wet patch?

 

[mrrusty]

Why would hygroscopic salts be concentrated down one corner of the chimneybreast,

Probably because the corner is a weak point in the surface between brick and room air. I am seeing exactly the same - salt deposits on the corner - I still have timber corners under the plaster. I know mine is not coming up from below, but from inside the chimney structure, because of where the salt deposits appear on the surface.

I have been renovating a 1902 house and it's been interesting to see how dabs also draw salts out the bricks. When you get "damp" patches in isolation 1.5m up the wall ONLY where the dabs are, the only explanation is the wet dabs have drawn salts out the wall which then draw humidity to the surface and then show up as a dark patch. I saw this phenomenon on internal walls in a room where a coal fire had burned every day for 100 years ( we know this because house was built by OH's great grandad).

 

[Red2k]

Thanks all, was busy pulling out the wardrobe to the left to figure out a potentially different issue (https://www.diynot.com/diy/threads/...bare-wall-over-suspended-timber-floor.557440/)

The good news is that the damp does not go around the corner to the side of the chimney breast, pretty much bone dry there. Have a moisture meter on order from Amazon and should be able to confirm when it arrives.

The lining paper was damp and easy coming off the wall, so to test my theory I pulled it away from the wall and dried it with a hair dryer. Left it for a few hours and it was wet again even though the paper was not touching plaster....so the salts must have made their way into the lining paper as well.

Bad news is that I have wet floorboards a couple of feet away from the chimney breast (per the other thread) and need help with that. Appreciate any thought on if I call a builder or a damp specialist?

 

[JohnD]

When you get "damp" patches in isolation 1.5m up the wall ONLY where the dabs are, the only explanation is the wet dabs have drawn salts out the wall

"only?"

the other, very common, explanation for damp patches on the dabs is that they are in contact with a cold external wall, so are cold bridges that attract condensation, or, if the wall behind is damp, it travels through the dabs.