damp coming from closed chimney breast

I have have a chimney breast in the dining room that has been closed up with plasterboard for years (before I moved in). When I repainted that wall recently suddenly what looks like sweat patches have appeared through the new paint up to 1 metre off the ground. I used an oil based undercoat but this didn't help. I don't really want to open up the wall so what is the best way to simply cover this up? Unless of course you think I really ought to tackle the cause...

Raphael

Reply to
lesshaste
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You are buggered mate.

Its one of two thimgs.

Rain coming down teh chimney - whuch could be capped if no longer used and saoking into teh lower brickwork, or as in my case, a fireplace that sits in an underfloor puddle under the house and is suffereing from extreme rising damp.

Chimneys are desined to be ventilated, and have fires lit in them, which normally pulls moisture introduced that way up the chimney.

The real answer is to cap the chimney, and put a DPC in the base.

You may care to investigate if water is getting under the house though, assuming it has raised floors.

Sometimnes a french drain around the property prevents ingress.

The soil below a house SHOULD be bone dry..no water falls on it after all, BUT if someone has built e.g/ solid paths around it that allow water to run off THROUGH the underfloor vents INWARDS, or simply that your local water table gets to be high after it rains, it isn't always so.

Reply to
The Natural Philosopher

I have had a problem with damp walls where the previous owner of the house had removed the chimney breasts and stack completely. What they didn't do was remove the foundations of the fireplace, but instead they filled it in with random rubble and such like which had bridged the dpc of the wall. May be somthing to bear in mind.

Reply to
hyte1

You may find the base of the chimney is full of soot, which seems to hold moisture well for some reason.

Reply to
Stuart Noble

It may be caused by rain coming in (eg roof leak, open chimneypot, flashing or fillets), lack of ventilation or salt contamination. First thing to check is youve got a vent grille in place, one should go in when its sealed off.

Trying to stop it coming thru with waterproof paint will simply prevent evaporation and thus is liable to make the patch gadually larger. Its not going to solve the cause.

NT

Reply to
meow2222

Not ruling out other causes like rain entry (and it's a good idea to put a ventilating insert in the chimney pot anyway), but it does sound very much like classic rising damp to me. The 1 metre high is the giveaway, plus it's very common on older chimney breasts without DPC's, even when the walls are unaffected sometimes.

I'm surprised nobody has asked if the chimney breast is on an internal or external or a party wall. If it's an external wall there could be some other defect that's relatively easy to cure, such as leaking gutters/rainwater pipes, poor pointing or the ground level is too high. If it's on an internal wall or on a party wall and next door is occupied, then it's highly unlikely to be serious condensation in the flue as this only forms on relatively much colder surfaces. In fact venting a flue from the room can actually transport vapour up the flue and *encourage* condensation in some circumstances.

Treating the wall surface with an impervious membrane (e.g. oil paint) will most likely drive the moisture higher until it finds somewhere it can evaporate. If you can live with the dampness then I would suggest you remove the plaster up to 1m high and replaster with a lime-based plaster. This will allow the moisture to evaporate more freely so it doesn't go as high up the wall and will be much less obvious. That's how the house was built in the first place. Whatever you do, you must get rid of any pink or grey gypsum plaster as it will soak up the moisture like blotting paper and then begin to crumble.

If you need to cure the dampness altogether, you'd be best installing a damp-proof course at floor level, but you'll still have to renew the plaster up to the 1 metre level.

Peter

Reply to
Peter Taylor

You can't cover it it -- it will just move somewhere else. Is the chimney on an outside wall? Is there a vent into the fireplace? Is the top of the chimney open or closed and/or vented?

Chimneys must be vented top and bottom or they steadily get more and more moisture trapped in them. I opened up a fireplace which had been blocked off for 20 years at the top, and it was wringing wet, with condensation running down the flue which can't escape. It took 3+ months to dry out having put a vent in the top and fully opened up the bottom. Capping the top without venting it is much worse than leaving it open, but it must be vented both top and bottom so there's a through-draft. If the chimney is on an outside wall, the bottom vent can be to the outside rather than into the room.

Reply to
Andrew Gabriel

If the chimney is capped, you don't have a vent in the room. Think about it.

Reply to
Stuart Noble

The cap should be vented.

MBQ

Reply to
manatbandq

How does that work then? What stops the rain blowing in?

Reply to
Stuart Noble

Rain coming in through the open pot isn't a problem -- a properly ventilated flue will dry that without any problem. Blocking the airflow through a flue on an outside wall is the problem. Moisture gets into the flue slowly through the brickwork. Without ventilation, it can't get out at the same rate, and you end up with condensation running inside the flue. It usually concentrates at the bottom, but you can get it coming though the wall anywhere. Flue must be ventilated top and bottom to prevent this. Capping at the top without ventilating will cause a damp flue.

Capping with a vent is the best option (particularly if a tall stack is exposed to the weather), and second best is just leaving the pot open. In either case, a vent at the bottom is required too. If the chimney is on an outside wall, the ventilation could be to the outside rather than to the room.

Reply to
Andrew Gabriel

It is a Victorian terraced house and the chimney breast is on what I think you call a party wall (a wall that two houses share in any case). There is a small air vent in the plasterboard if I remember correctly although it could of course be blocked. I really don't know what the situation is with the top of the chimney as it would involve going on the roof and there is no easy access without a very tall ladder.

The thing that is frustrating me is that before repainting there was no obvious problem. This whole expensive looking problem seems to have just appeared by repainting!

If I really have to have the thing opened up and treated by a "professional" does anyone have a recommendation (e3, east london) for someone who isn't a crook? I had a very bad experience last time I tried to hire a damp proofing company.

Raphael

Reply to
lesshaste

one of the reasons we diy

NT

Reply to
meow2222

You only need a local odd job man really.

Reply to
Stuart Noble

A properly designed vented chimney cap will let very little rain in. What does get in will simply dry when it isn't raining.

MBQ

Reply to
manatbandq

DIY the damp proofing then you know its done and done properly.

I havn't tried it yet but have just ordered this:

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soon as it arrives I'll be damp proofing around our old fireplace. I don't think it needs it as per my last post but im doing it anyway as we are paying for the whole room to be re-plastered and what to make sure the damp problem is solved.

There are some alternatives, dryzone is another varient of the cream as above but is more expensive. The other method involves pressure pumps which still is within DIY but a little more work.

Chris.

Reply to
hyte1

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I hope your confidence isn't misplaced. Might be easier to sort the source out.

Reply to
Stuart Noble

Problem has been sorted - previous owner removed fireplace and chimney breast but left in foundations of fireplace under floorboards. The gap was filled with random rubble and sand etc which I have now removed. I'm putting in the damp proof cream as an extra measure although I suspect I don't really need to.

I thought that seen as the floor is lifted, its worth =A35 a tube of damp proof cream.

Cheers.

Reply to
hyte1

Before calling someone in I'd try a couple of basic things:

Firstly are you sure that the vent isn't blocked? Is there any way that you can take it off & have a look?

Secondly if it was all OK until you painted it with an oil based paint that suggests that in the past it was getting enough ventilation to stay dry - is there any way that you can remove it and see if that returns it to it's previous state? (Obviously if the plaster's shot you may have to replace it anyway).

Michael

Reply to
michaeld121

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