Damp near chimney breast (Ed.)

[Edislaw]

Good Morning and thank you for allowing me to join te forum. I think I might be quite active over the next few weeks as I work to bring our new property, (soon we hope, our forever home) up to a comfortable standard. We live in the rural south of Scotland where installation services are scarce and winters can be challenging. For background, our property was built in at least five stages over one and a half centuries. The last but one owner installed an Air Source Heat Pump but was ill advised as no insulation was installed with it. Curiously, that may have done us an expensive favour as it forced a much closer examination of the fabric than might otherwise have been the case.

My first question to the forum relates directly to the comment in the Home Report for the property that damp readings were high near the base of a chimney breast. It was only when I started to investigate that it became clear the the Home Report left something to be desired, indeed, many things! One issue missed was the four chimney breasts in the property did not match the three chimneys on top. Had the surveyor noticed this he might also have noted the chimney breast with no chimney was the one which demonstrated high damp levels.

Yesterday we managed to reach the top of the chimney breast in the loft and it looks (haven't been able to get a camera completely over the chimney yet} as if the chimney is open. while the loft is apparently quite dry it is. of course, unheated and our weather in the south of Scotland this year hasn't been exactly dry. In addition, the report was written in January with snow on the roof and the house has now been empty ofr ten months. The bottom of the chimney lies above a framed and dry lined faux fireplace with an electric fire fitted into it.

We are in the process of planning the installation of underfloor heating so the room will be properly heated. We plan to dispose of the electric fire, frame and dry line the faux fireplace over its face. I have three questions:

1. Should we block and seal the chimney top and bottom?
2. Should we fill it and if so with what
3. Is it a reasonable assumption that proper heating, particularly the steady heat of UFH. will allow the damp (which is not visible) to dry naturally?

 

[JohnD]

I don't know what your first pic is. Stand back and take a wider one to show context and scale.

Don't block the chimney or it will get internal condensation. Ventilate each flue top and bottom.

I think you mean that you have a disused chimney that is damp at the bottom.

This is quite common and there are some usual causes

The hearth does not have a DPC so it wicks up water from the ground. This did not matter when constant fires kept it warm and dry

The disused chimney does not have constant airflow up it to ventilate away the damp

Lazy builders commonly throw the rubble into fireplaces and under floors to save themselves the effort of carting it away. The rubble holds damp.

Simplest remedy is to look in the fireplace and remove any rubble, add ventilation top and bottom.

Best remedy, if you can do building work, is to open up and dig out the hearth, remove the rubble and soil under it, will probably be two or three feet deep to the bottom of the chimneybreast brickwork. Clean, exposed brickwork will not easily carry damp this far, as it will evaporate off the surface if ventilated. Let the cavity ventilate to the subfloor void (remove any subfloor rubble) and the flue so it can dry out. If you want an exposed hearth, add a dwarf brick wall to support a couple of paving slabs, or cast a concrete pad on the brickwork with DPC to prevent damp transmission, and an airspace beneath it so it is not on damp soil.

There is a faint chance than an old drain nearby is broken and making the ground wet, or that rain is falling down the chimney, look out for any signs of these.

Warmth does not remove damp, you need to ventilate it out of the house.

 

[Edislaw]

The first picture shows the top of the chimney which is below the roof ridge, I should have explained that more clearly.

The house is a single storey cottage so the chimney is around 5m top to bottom. I should also have pointed out that this is an internal chimney breast. Luckily, the ground is very dry and well drained as there is virtually no sub floor. There are no services anywhere below the building.

I have read that filling the void with vermiculite can solve the problem, any comments very welcome.

 

[JohnD]

Blocking will block ventilation, so will make any damp worse.

If the top of the chimney has been partially demolished so it is open inside the loft, I've found that keeps them dry, but you seem to have a source of damp at the bottom, most likely coming up from the ground as described.

 

[Edislaw]

Thank you, that certainly seems logical. I'll investigate that idea.

 

[mrrusty]

Blocking will block ventilation, so will make any damp worse.

How many times! - A chimney which is open at the bottom in a warm humid ambient living space and open at the top in a cold roof void is a condenser.

Warm air will rise from the living space up an open chimney, not only because warm air rises because it is less dense, but because, everything else being equal, warm air is at a higher pressure than the same air at a lower temperature because of the increase in vapour pressure with temperature. When that rising warm air cools to the dew point in the chimney in the cold attic, the water vapour condenses out. Maybe enough even to run down the inside of the chimney.

We have to deal with this phenomenon routinely in hospitals served by pneumatic tube transport systems (160mm pipe) through roofs. Some hospital rooms tend to be pressurized for cross contamination reasons, and if we don't control the air going up the pipe to a cold roof, we end up with literally gallons of water on the floor by the pneumatic tube station.

 

[Edislaw]

thank you, that's a very clear explanation. If, therefore, we are able to stop the warm air from the room entering the chimney, perhaps by sealing the chimney withj a plasterboard covered frame backed by solid insulation, is that likely to solve the problem?

 

[mrrusty]

That's what I would do - if the top is open in the loft space, then I'd seal the bottom. The other risk is that nice warm air gets in to the roof void and condenses on the cold rafters and underside of the tiles/slates. This problem in older houses is exacerbated of course by loft insulation making roof voids comparatively much colder. If you are going to vent a chimney at the bottom, do it to outside, that way condensation risk is eliminated.

 

[JohnD]

it needs to be ventilated top and bottom.

In some cases, you can let a disused chimney draw air from the void beneath the floor.

This chimneybreast is already damp at the bottom.

It is an internal chimney so the brickwork will approximate to room temperature.

 

[Edislaw]

Thank you but ventilation to the outside is a little difficult as this is a chimney breast in the middle of the house. but it appears to have been done, at least partially correctly, we just need to check the insulation at the bottom. The loft is well ventilated and condensation does not appear to have been a problem. I suspect the underfloor area is the culprit!

 

[mrrusty]

No, it needs to he ventilated top and bottom.

I think we are going to have to disagree on that one. This is janet and john physics- ambient air at 20 degrees 60% RH has a dewpoint of 12 degrees (see the table - sat VP @ 20 degrees = 17.54. 60% = 10.52 = saturated VP at 12 degrees) If the top of the chimney is colder than 12 degrees that ambient air rising will be condensing at the top. If the air at the bottom going in was 25 degrees and 70% RH you'd have had 6g of condensate per m3 of moved air when cooled to 12 degrees.

 

[mrrusty]

damp readings were high near the base of a chimney breast

This is a completely different issue (or at least there is a different topic which also influences damp readings). Damp conducts electricity, and damp meters are resistance meters. However, salts also conduct electricity, so a salt-poisoned brick will appear "damp" to a damp meter. So damp readings can be misleading

Salts arise because nitrogen and sulphur combustion products from burning coal react with water to form weak nitric and sulphuric acids, which then react with calcium carbonate (lime, limestone) and other building products to form salts, which mostly are also hygroscopic and draw water vapour out the air and grow crystals. (Same as acid rain eating limestone buildings). Always lots of salts around chimneys which can produce symptoms of damp even when there is no penetration or condensation - the water all comes out the air in the room, drawn in to the salts.

 

[Edislaw]

Fascinating information but I suspect the most likely candidate is salts in the plasterboard.