Damp on hidden fireplace

[Ivor Windybottom]

This is what I did with my two redundant very tall chimneys...

No leaks, damp patches or worries ever again.

 

[aveatry]

Vents don't have a one-way valve. In theory the warmer air should rise up the chimney so act as an extractor. But the wind blows, anything can happen. We had a vented fireplace that would stink of old chimney when I put the kitchen extractor on 3 rooms away. Modern houses have little ventilation and various extractors, air needs to get in somewhere and a vent is the most likely place. Even if you have slot vents on the windows the chimney vent is likely to be the easiest route.

Even if the air does exclusively flow from the room into the chimney, that's a waste of expensive heat and that air has to be replaced from somewhere - colder air must be entering the living space from somewhere - e.g. under a door, through gaps in the floor etc.

I'm not convinced by the idea of using warm, damp internal air to ventilate a chimney. It's the simplest way, and how it was done in the past, but that was when heating was cheap.

If the chimney has an outside wall then venting to the outside at the bottom is a much better way. But getting rid of a disused chimney completely is a far better idea, otherwise they're a liability without any positive benefits.

Talking about underfloor air flow. I thought every house had a prevailing "route" due to the prevailing winds and air pressure and orientation of your house. I am not talking about wind pushing in the outside vent its more like the wind causing negative air pressure on one side and so air is sucked in. Don't they say that a brick wall is not blown over by the wind but rather it is pulled over by negative air pressure on the other side..
If there was no prevailing direction of the air flow in the floor void then the air would not "bother" to go in the brick vent at the side of your house in the first place.

 

[aveatry]

Don't feed the troll. The thread's about a chimney.

The dirty debris issue. I was getting to the point that if he is in the states then he would not likely have come across chimneys with hundreds of years worth of coal soot. Some of our houses are older than the country he lives in.
And his avatar location says California - I am sure they have lots of coal fires there.

 

[Ivor Windybottom]

Talking about underfloor air flow. I thought every house had a prevailing "route" due to the prevailing winds and air pressure and orientation of your house. I am not talking about wind pushing in the outside vent its more like the wind causing negative air pressure on one side and so air is sucked in. Don't they say that a brick wall is not blown over by the wind but rather it is pulled over by negative air pressure on the other side..
If there was no prevailing direction of the air flow in the floor void then the air would not "bother" to go in the brick vent at the side of your house in the first place.

I think you're overthinking it! If you stick a hole in something the air can go either way.

Outside air does all kinds of weird stuff. There my be a prevailing direction but it's pretty random, and there's usually turbulence from the buillding and others surrounding it that can make anything happen.

 

[Ivor Windybottom]

The dirty debris issue. I was getting to the point that if he is in the states then he would not likely have come across chimneys with hundreds of years worth of coal soot. Some of our houses are older than the country he lives in.
And his avatar location says California - I am sure they have lots of coal fires there.

Definitely overthinking! Best not to care.

 

[Harry Bloomfield]

Vents don't have a one-way valve. In theory the warmer air should rise up the chimney so act as an extractor. But the wind blows, anything can happen. We had a vented fireplace that would stink of old chimney when I put the kitchen extractor on 3 rooms away. Modern houses have little ventilation and various extractors, air needs to get in somewhere and a vent is the most likely place. Even if you have slot vents on the windows the chimney vent is likely to be the easiest route.

I am not entirely convinced by the suggestion of condensation happening inside the chimney - though may depend upon where in the building, the stack is located...

We have two fireplaces, one on the first floor, vented cap, and a vent where the fireplace used to be, then one on the ground floor, fitted with a gas fire, and lined. Both rise via a single stack, built alongside an internal wall.

After the above was done, and the chimney capped, and for some years after, we would see a brick pattern appear on the wall paper, on the upstairs chimney breast. Later redecoration hid that, and it has not reappeared in the years since. My assumption was that the brick marks appeared, due to old moisture in the chimney, gradually drying out.

I've never noticed any cold air falling down, or any detectable warm air flowing up the downstairs chimney, except when the gas-fire is in use. Wind, and we get some really strong winds here, seem to not make any difference to how the fire burns.

 

[Harry Bloomfield]

Outside air does all kinds of weird stuff. There my be a prevailing direction but it's pretty random, and there's usually turbulence from the buillding and others surrounding it that can make anything happen.

An apex roof, leading to a chimney stack, which is the most common design, will act like a wing, normally causing a vacuum, above the pot.

 

[mrrusty]

will act like a wing, normally causing a vacuum, above the pot.

It will, and is why fires draw better when it's windy. The same force draws warm room air up a chimney if it's not in use........

 

[stuart45]

Downdraughts in chimneys can be caused by trees that are higher than the chimney close by.

 

[JessWare]

Vents don't have a one-way valve. In theory the warmer air should rise up the chimney so act as an extractor. But the wind blows, anything can happen. We had a vented fireplace that would stink of old chimney when I put the kitchen extractor on 3 rooms away. Modern houses have little ventilation and various extractors, air needs to get in somewhere and a vent is the most likely place. Even if you have slot vents on the windows the chimney vent is likely to be the easiest route.

Even if the air does exclusively flow from the room into the chimney, that's a waste of expensive heat and that air has to be replaced from somewhere - colder air must be entering the living space from somewhere - e.g. under a door, through gaps in the floor etc.

I'm not convinced by the idea of using warm, damp internal air to ventilate a chimney. It's the simplest way, and how it was done in the past, but that was when heating was cheap.

If the chimney has an outside wall then venting to the outside at the bottom is a much better way. But getting rid of a disused chimney completely is a far better idea, otherwise they're a liability without any positive benefits.

Unfortunately all the chimneys are internal walls.

I'd really like to keep it simple if possible and avoid any more damp getting in to the walls. The fireplace with damp is in a hallway so I don't want to expose it either although I am impressed with the examples shown.

The house does have a void underneath but I can't see any venting into the void, or the walls apart from vents for fan extractors. There's also no vents on the double glazed windows.

I take it that the first step would be to get the chimneys capped and see if that sorts the damp out before opening up any walls, is that right?

 

[aveatry]

Unfortunately all the chimneys are internal walls.

I'd really like to keep it simple if possible and avoid any more damp getting in to the walls. The fireplace with damp is in a hallway so I don't want to expose it either although I am impressed with the examples shown.

The house does have a void underneath but I can't see any venting into the void, or the walls apart from vents for fan extractors. There's also no vents on the double glazed windows.

I take it that the first step would be to get the chimneys capped and see if that sorts the damp out before opening up any walls, is that right?

Mr rusty is the expert but I would say yes - cap first - a cap that allows air flow. But you are on the ground floor - I would have thought that any rain ingress would of hit the sides further up stairs and show in the bedroom.

 

[Harry Bloomfield]

The house does have a void underneath but I can't see any venting into the void, or the walls apart from vents for fan extractors. There's also no vents on the double glazed windows.

You must have some through ventilation, under the floor, to help keep timbers dry, and prevent them rotting. Check around the base of the walls, outside.

Yes, the priority ought to be, to prevent rain water, getting in the chimney by capping with a vented cap, followed be venting at the base.

 

[mrrusty]

Mr rusty is the expert

I'm not. All I can offer is the science behind condensation and humidity that was part of my Merchant Navy training back in the day, and my own personal experience working and renovating a couple of 100+ year old houses and noting what has been successful and what not, and trying to work out the science behind what I have seen.

Based on my own practical experience, I think ventilation is actually the lowest of four factors causing damp in/on chimneys, which IMHO are:-

1) rain coming down the chimney
2) damp patches caused by salts resulting from the chemical reactions of coal fires, water and building materials like lime
3) penetration from roof leaks around flashings etc.
4) ventilation

My guess here is that these damp patches are salts, which is why I linked to an article about this. I have personally cured this problem very succesfully by stripping back to brick, isolating the salts with a cement/SBR slurry and then fixing plasterboard with foam adhesive. I have also found that wet plaster and wet dabs applied to salt poisoned bricks allows the salts to migrate to the surface. The key is isolation - stop the salts migrating outwards as any moisture evaporates from the surface, and stop room humidity reaching the salts in the wall.

If you do have salt-caused damp patches, ventilation will do nothing - it could even perhaps make it worse by feeding the hygroscopic salts with more water vapour. Ventilation has one sole purpose - to evaporate any liquid water.

I'm sure others have different opinions based on their experiences, and their opinions are as equally valid as mine.

 

[stuart45]

Damp issues in chimneys are caused by a variety of things. Whether it's an external or internal breast makes a difference. the height, size, design, materials used, whether it's lined or parged, and condition of the stack will also affect it. As with any damp problem, the first step is to identify the cause. Sometimes it's more obvious than others. My experience of working on chimneys is to start at the top for the analysis and work your way down. Sometimes it can be a combination of issues, so fixing the flaunching, repointing the stack and replacing the flashing only partially fixes the problem.

 

[Ivor Windybottom]

I'd look at capping the top, wait a year then see if the problem goes away, then look at venting from the bottom into the floor void. It's essentially outdoor air down there, without the heat and moisture you'd get in indoor air*. Plus you won't be extracting heat from the room. Arguably it could make the chimney walls colder in the rooms, but I wouldn't expect a major difference from a trickle of already partly heated air.

* If it's dry. Some sub-floors can flood, especially if the drains aren't so good. There's a whole other can of worms to open if you start looking at it.