One of my chimney breasts has a few smallish damp patches spoiling the decoration (painted plaster) and I seek opinions on how best to deal with it prior to redecorating.
I'm pretty sure it's condensation onto salts in the brickwork from fires over the years.
So what do I do to it?
TIA
Jim K
Hi Jim,
from the experience I have; if the stack alone is 'sweating', it is most likely that the ventilation in the flue is limited, if not zero. Possibly, the stack has been capped with little to no venting.
If the ceiling(assuming upper floor) and breast is wet, then most likely a stack to roof bond is compromised. ie., lead flashing or soaker is misplaced, Else, mortar needs re-pointing.
...Ray.
Down low or higher up? Higher up is usually rain onto stack brickwork lower down may be rising damp.
Either way the remedy for your purposes is to make sure the damp goes to where it wont be harmful - e.g. the stack interior. The way to do that is to cut out any blown plater, for a large area around the actually damp area and use a waterproof render coat, and re-skim that...
In response to queries....
The affected areas are several maximum hand-sized areas on a ground floor chimney breast, around 6 to 7ft from the floor.
The flue behind the affected areas is lined and backfilled with vermiculite insulation.
The current plaster is very thin but not blown just damp.
Is there anything I can 'paint' on? I tried Thompson's dampseal but unless my old tin had gone off, it didn't do anything.
Jim K
This is classic condensation in the flue, due to lack of ventilation.
Is the flue still used?
What's the construction, and does it back onto an outside wall? What's the state of chimney flaunching?
Urgh - don't know how you'd ventilate that now. I suspect the vermiculite is partially waterlogged, and it can hold very large amounts of water.
Plaster (gypsom) dissolves in water, albeit slowly. The layer that bonds to the wall will simply come away if it hasn't already. Painting anything on the surface would only be a short term measure, and might actually speed up the plaster disintegrating by keeping it wetter and forcing the moisture to spread further out to escape.
Amen to that. Rain down the chimney perhaps?
/Amen to that. Rain down the chimney perhaps? /Q
You did read the bit about 'lined' didn't you?
Jim K
/This is classic condensation in the flue, due to lack of ventilation.
Is the flue still used? /Q
Yes occasionally /What's the construction, and does it back onto an outside wall? /Q
Brick, and yes it's on a gable end.
/What's the state of chimney flaunching?\q
Pass but top of stack was rebuilt approx 5 yes ago.
/> The flue behind the affected areas is lined and backfilled with vermiculite insulation.
Urgh - don't know how you'd ventilate that now. I suspect the vermiculite is partially waterlogged, and it can hold very large amounts of water./Q
Possibly though it's summer and why do I only have a few patches showing on the gfloor breast ?
Jim K
Doesn't mean rainwater isn't finding its way into the insulation
+1
Rain into the pot should only go down the inside of the flue liner.
Rain on the outside of the pot will run down to the flaunching and if that is cracked/damaged it get into the space between liner and chimney now filled with nice absorbant vermiculite...
/+1
Rain into the pot should only go down the inside of the flue liner. /Q
Indeed.
/Rain on the outside of the pot will run down to the flaunching and if that is cracked/damaged it get into the space between liner and chimney now filled with nice absorbant vermiculite... /Q
Mmm thinking and goggling I see that vermiculite is dark brown?, Istr I used perlite, which is a lot lighter in colour & more mica like.
However I will have to get on the roof to look at flaunchings.
But why have I only got 5 damp palm print sized patches?
Jim K
I agree, sounds like the Vermiculite has got wet. Not likely to be condensation this weather. Esp. if the chimney is insulated. So you have to find out where the water has got in, first job. Could be duff pointing, flashing or the haunching round the terminal/faulty terminal. As for the damp patch(es), suck it and see. If you can fix the leak it may just go away. The water is coming out at a weak point or there may be a ledge inside the chimney (esp if it is not entirely vertical).
There's likely a (usually horizontal) plate with a hole in it (for the liner) at the lowest extremity of the chimney/behind the heating appliance. Keeps the vermiculite in. Check if there's any damp/rust etc there.
'cause you have five pathways from the damp source to the surface. Like anything flowing it take the path of least resistance though it's not that simple with damp. If there is a place that evaporates slightly faster than the surroundings there will be a "pressure" that will tend to push more damp to that area.
/cause you have five pathways from the damp source to the surface. Like anything flowing it take the path of least resistance though it's not that simple with damp. If there is a place that evaporates slightly faster than the surroundings there will be a "pressure" that will tend to push more damp to that area. /Q
Mmm theories abound... so why isn't the least resistive path the untouched original brickwork that forms the south facing outside of the chimney breast - that is warm to the touch (from the sun) still this evening?
Jim K
evaporates
Probably because the bricks get rained on and thus consideraby wetter (hopefully!) than the inside of the chimney. Inside will be always dry and dryer than the inside the chimeny so damp will move to wards the inside.
Need to think in terms of gradients with damp flowing from wet to dry, like heat flows from hot to cold. Lots of things affect the gradient and the resistance to flow that the materials have.
A fired but unglazed ordinary clay pot water will drip through, line it with a bit of cling film no flow at all, same gradient different resistances to flow.
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