Damp on hidden fireplace

[JessWare]

Thanks everyone! So do you think I should lift off all the plasterboard to look for an airbrick in the (hopefully) bricked up fireplace? Or could I get away with just putting a vent on the plasterboard?

I've been in touch with the precious owners and they've no idea there were fireplaces there apparently, and had no issues for around 10 years.

 

[aveatry]

Once you've made sure the chimneys are capped with vented cowls (if they're not, a lot of water will be coming in - think how much rain accumulates in a bucket left outside) and then read this from a damp specialist company https://www.tracebasementsystems.co.uk/post/damp-chimney-breasts

Ventilation can work, but only if no part of the chimney is colder than the dew point of the air you are letting in. You can use a hit/miss vent and take the time and trouble to assess whether there is a condensation risk at that time and open/close the vent accordingly, but on balance because the dew point of room air is about 12 degrees, and the loft/top of chimney is very often colder than this, you have to be very careful with through-flow ventilation because you can introduce more problems than you solve. If it's 15 degrees or warmer outside, yes, ventilate, but if it's not, then don't.

The best place to ventilate a chimney for through-flow is to outside, because the air being drawn in is already at ambient temperature, and as there is no scope for further cooling inside the chimney you won't get condensation.

To those who still persist in suggesting that bottom ventilating a chimney is always a good idea, I ask "why do we fit condensation traps at the bottom of vertical air extractor chimneys?".....https://www.extractorfanworld.co.uk/blogs/condensation-traps

Not easy in a semi. Would this work with a suspended floor, Add an air brick to external wall and connect a 110mm drainage pipe to it and run it under the floor to then come up inside the old fire place and blank off above- a bit like this picture but without the fire - just the pipe.
Or run it to the back of the opening and stop at floor level and have a false wall at the back of the old fire place that accommodates an air gap so that the pipe will not be seen.

 

[mrrusty]

You can do anything that works. The fundamental fact is that nice warm fairly humid room air at ~20-22 degrees and 50-60% relative humidity will start condensing to liquid water at around 12 degrees.

If the air coming in from outside is say 14 degrees and 80% humidity because that's the temperature outside and it's a damp day, that air still has a little capacity to evaporate liquid water provided it doesn't cool any more.

Air that isn't saturated can always evaporate liquid water.

Controlling the conversion of water vapour in air to liquid condensate is not specific to chimneys. It applies to anywhere there is a temperature gradient with a dew point somewhere along that gradient. My training was all in the merchant navy where this is/was (particularly before containerisation when cargoes were carried directly in holds) a massive issue, and condensation control was critical to not destroying the cargo.

 

[aveatry]

You can do anything that works. The fundamental fact is that nice warm fairly humid room air at ~20-22 degrees and 50-60% relative humidity will start condensing to liquid water at around 12 degrees.

If the air coming in from outside is say 14 degrees and 80% humidity because that's the temperature outside and it's a damp day, that air still has a little capacity to evaporate liquid water provided it doesn't cool any more.

Air that isn't saturated can always evaporate liquid water.

Controlling the conversion of water vapour in air to liquid condensate is not specific to chimneys. It applies to anywhere there is a temperature gradient with a dew point somewhere along that gradient. My training was all in the merchant navy where this is/was (particularly before containerisation when cargoes were carried directly in holds) a massive issue, and condensation control was critical to not destroying the cargo.

So on balance is it better to have an inside vent in the old fireplace and a capped chimney (with air flow vent) than no venting at all.

 

[mrrusty]

My 2p on this is in order from best to worst

1)Vented at the top, externally vented at the bottom - air flow with virtually zero condensation risk in all conditions
2)Vented at the top, not vented at the bottom. The open top will still allow evaporation of any dampness in the chimney caused by salts and/or penetration, but evaporation will be slow as there is little air movement.
3) vented at the top and bottom internally with a hit/miss vent that is used occasionally when the conditions are right so that condensation risk is controlled
4) permanently vented internally at the bottom and you just take a chance that there are more "drying" days than "condensing" days, or you just open in summer and close in winter.

 

[aveatry]

My 2p on this is in order from best to worst

1)Vented at the top, externally vented at the bottom - air flow with virtually zero condensation risk in all conditions
2)Vented at the top, not vented at the bottom. The open top will still allow evaporation of any dampness in the chimney caused by salts and/or penetration, but evaporation will be slow as there is little air movement.
3) vented at the top and bottom internally with a hit/miss vent that is used occasionally when the conditions are right so that condensation risk is controlled
4) permanently vented internally at the bottom and you just take a chance that there are more "drying" days than "condensing" days, or you just open in summer and close in winter.

A much needed answer covers everything.

 

[aveatry]

Various responders have offered the most silly & complicated bits & bobs of inexperienced replies.

I've previously installed or inspected lots of vented flues, & had never seen any sort of baffle behind fixed vents or Hit&Miss vents.
Neither have I seen debris & soot coming down a swept flue with a top cowl, & out through the bottom vent.

In redundant flues the bottom vent is internal and the top is typically a redundant flue ventilating cowl.

What country are you in - is it USA - as your flag.

 

[Ivor Windybottom]

The best option of all would be to get rid of the chimney completely and roof over where it was. Then you don't need to worry about rain getting in, decaying flashings, ventilation etc at all, it's then basically just a dry cavity.

If you really wanted to, and could coordinate with willing neighbours, you could remove it from top to bottom and gain some interior space.

I really wouldn't want to ventilate a chimney into the room. It's how it was done in the past, but we did lots of other terrible things in the past too. You're creating one horrible cold draught right into your room that probably stinks and contains all sorts of nasty stuff from the soot in there. Plus you're letting warm, damp air from your room into the cold chimney, where that moisture will immediately condense into liquid water.

 

[Ivor Windybottom]

A possible off-the-top-of-my-head crackpot idea would be... if no major work is an option then could you vent it into the floor void?

Whatever, it definitely needs work outside if the rain is getting in. Even if it's just a cap and looking at the flashings.

 

[aveatry]

The best option of all would be to get rid of the chimney completely and roof over where it was. Then you don't need to worry about rain getting in, decaying flashings, ventilation etc at all, it's then basically just a dry cavity.

If you really wanted to, and could coordinate with willing neighbours, you could remove it from top to bottom and gain some interior space.

I really wouldn't want to ventilate a chimney into the room. It's how it was done in the past, but we did lots of other terrible things in the past too. You're creating one horrible cold draught right into your room that probably stinks and contains all sorts of nasty stuff from the soot in there. Plus you're letting warm, damp air from your room into the cold chimney, where that moisture will immediately condense into liquid water.

When you put a vent in it doesn’t go into the room - the air flow is from the room into the vent.

 

[aveatry]

A possible off-the-top-of-my-head crackpot idea would be... if no major work is an option then could you vent it into the floor void?

Whatever, it definitely needs work outside if the rain is getting in. Even if it's just a cap and looking at the flashings.

That would cause a "short circuit" in the floor void air flow so instead of full front to back air flow (or the prevailing air flow direction ) it would only go to the fireplace, half way in on room. That' why I said add an air brick and use a 110mm pipe.

 

[aveatry]

Poster #25,
Thank you for the inquiry: turning left at Iceland & keeping on going should find the flag you mention.

So are you working and posting on here from Canada or America

 

[Ivor Windybottom]

When you put a vent in it doesn’t go into the room - the air flow is from the room into the vent.

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.

 

[Ivor Windybottom]

So are you working and posting on here from Canada or America

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

 

[stuart45]

Venting a chimney from the inside or outside depends on the situation, and neither is perfect for every situation. Venting from the outside can cause cold spots on the inside wall which will attract condensation. Venting from the inside can take damp air from the room into the flue. A chimney really needs a proper assessment before it's done, but this rarely happens. If one method from better than the other in every situation it would be well known by now.