damp and sound/heat insulation of internal wall

[steveb]

(repost from greenbuildingforum)

I have a wall that is causing me much head scratching - any wisdom is appreciated. I also guess a picture tells a thousand words so there's a crude plan and a photo here :

http://s712.photobucket.com/albums/ww123/snyggapa/

The problem we have is with the party wall, pictured (centred on one of the chimney breasts) . It's 200+ years old, single skin brick , no DPC. Apparently it has a "damp problem" - neighbour has apparently had it drilled and injected 25 years ago, but that is now failing (or the waterproof cement they added to cover the failure has now failed) - either way, they apparently have damp against that wall in their property and there is no obvious internal source of the water.

To be fair, the wall kind of "looks" damp although some of that tide mark visible is from where the soil under the now removed rotten wooden floor has been excavated away. We have removed the coating from the wall - it used to be lime rendered , with a nice cement render and plaster on top, which I guess trapped the damp in nicely - and I would guess that the now exposed wall is now drying out quite effectively. The problem I have is what to do with it..

Unfortunately we can't just leave it exposed which would be my personal preference - it's so thin that you can hear everything next door - and the place is so small that you can't get away from the noise, so we need some sound proofing and if it comes with thermal insulation, all the better.

I am assuming that the damp is being drawn up from the ground - we're on a drained fen so have a high water table - and I have 2 choices , either to stop it rising by putting in place a DPC of some kind, or let it evaporate as originally intended.

We are hoping to be allowed to put back a suspended timber floor however this need to be agreed with the BCO since it violates pretty much every current rule regarding timber (flood plain, high outside ground levels) but if we could do that then it should be possible to leave the bottom 9" or so of the wall below the finished floor open in the ventillated void - is that likely sufficient to evaporate off the rising water, or would the damp typically rise to higher than this? Are there any ways of giving a helping hand by for example lime rendering the bottom 9" of the wall to increase the evaporation area, or would that likely just wick more moisure out of the ground?

If the damp is likely to rise above the floor then i'd need either a void between the wall and the soundproofing/insulation, or some kind of soundproofing/insulation that actively lets the water out of the wall. Does anyone have any ideas or better suggestions..

The domesday scenario which our plans agent is working towards is the installation of a concrete floor to current building regs standard - he recons this is all that BCO will approve - however I can only see this making the damp worse by trapping & forcing it it to the wall and giving it nowhere to evaporate to.

Does anyone have any suggestions how to approach this?

Thanks

-Steve

 

[Matt Green]

Welcome!

What's the height of your neighbours' floor in relation to the solum on your side of the wall? Also, are you willing to consider dry-lining the wall on your side?

M.

 

[steveb]

well, not entirely sure what solum means but the neighbours floor (renewed in concrete in the 1980s apparently, yuck) as far as I can tell is at about the same height as our intended floor finish and the external ground level - all the same..

no objection to dry lining that wall, it will need a "finish" of some sorts and dry lining is no problem. The only condition is that we would like the chimney pillars to remain exposed - the breast on the left we intend to keep just the pillars on view and put a small electric range cooker in the opening, the breast to the right will have a fireplace face rebuilt with lime mortar and the bricks we reclaimed and house a mostly decorative wood burning stove

Thanks

-Steve

 

[Matt Green]

Solum is the top layer of soil that a building sits on, or definitions close to that. It's used to refer to the soil under a floor.

Tricky situation. Any water trapped under your neighbours' concrete floor will move into the wall and it doesn't sound like it's going to entirely evaporate under the floor, given previous plaster damage and the high water table. It you leave the chimney breast exposed water will move into them and the bricks will spall (flake, turn to dust, as the water movement brings ground salts with it that are deposited on on just under the internal brick surface), especially if you have an oven in the opening. Lime plaster will result is constant tide marks and loss of finish at low level (IMO, given the information on the construction).

Dry lining the wall and leaving the chimney breasts exposed could give you problems with spalling bricks in the chimneys, but might not be too bad as some of the water in the wall will come out into the lining void. I think I would be inclined to take that approach.

 

[Penners]

Steve - just so you're aware, Matt is an expert in this field.

I'm not! But that's never stopped me from making a fool of myself with amateur suggestions!

On the assumption that you are obliged by the BCO to install a concrete floor, if I were you I think I would try to turn this to my advantage. On your side of the party wall I would construct an entirely independent new wall in concrete blocks, leaving an air-gap between it and the old wall. The more massive your new wall is, the better it will perform at sound attenuation, but if you cared to back it with sound insulation material (taking care still to leave an air gap between the insulation and the old wall) so much the better. And back the insulation material with a damp-proof membrane. So, in summary, working from your side you have concrete block wall, insulation material, membrane, air gap, old wall.

I would insert a DPC in the bottom of the new wall, ensuring that it had continuity with the DPM under your concrete floor, and a vertical DPC between the new wall and the chimney stacks.

I think I would also insert air bricks in the exterior walls, within the air gap, to allow it to be cross-ventilated.

It's essential to create the best possible air seal between your new wall and the air gap. Sound transmits through air, so you have to try and ensure that there is absolutely no air leakage between the sounding board that is the old wall, and your room.

Obviously this solution is going to lose you some floor space. But (subject to being shot down in flames by real experts!) I believe it would be the best solution, given your circumstances.

 

[Matt Green]

Steve - just so you're aware, Matt is an expert in this field.

I'm flattered, but any knowledge about water movement in buildings I have has been gained by proxy during the pursuit of learning more about timber decay organisms. Your proposal looks perfectly workable Penners, although:

I think I would also insert air bricks in the exterior walls, within the air gap, to allow it to be cross-ventilated.

Why would you need to ventilate? There's nothing in the void between the walls that can decay (any embedded timbers excepted). Keeping the air in the void at high humidity would prevent water evaporating from the party wall preserving its surface too.

 

[steveb]

Thanks Penners. re the concrete block wall, I can see that it could work but i'd much rather not have to do it. Also with a floor plan of 6mx7m i'm struggling for space so thinner is better I think we need that meeting with the BCO to set the parameters soon - apparently they passed the submitted plans today, so task #1 is now to get them to visit and change their minds or agree that these works do not come under building regs!

matt - now I understand re solum - maybe a couple more amateur drawings will make it easier.

This is a kind of a section along the plane of the photo:
http://i712.photobucket.com/albums/ww123/snyggapa/cross_section.jpg

Photo:
http://i712.photobucket.com/albums/ww123/snyggapa/IMG_1634.jpg

My hope is that we can get the BCO to agree that we can put in place a replacement timber floor over the bare clay earth, the construction would look something like this:

http://i712.photobucket.com/albums/ww123/snyggapa/cross_section2.jpg

so digging out and gravel filling the external ground whilst putting back a new timber floor at about the same height as external ground level. This would leave a ventilated void under the floor (need to measure the depth but in the photo looks like about 2-3 bricks, roughly 8" below FFL , we could perhaps dig out more if prudent) and hence 8" of the damp wall would be exposed in the void.

Is this a sensible approach, and if so would 8 or so inches of exposed wall be likely enough to naturally move the damp from the wall? The void under the floor was previously filled up with the filth of the ages until it met the underside of the floorboards, so ventilation was "somewhat limited" hence I guess a damp problem! Most of our structural timber has rotted away to at least 2 feet from FFL.

This is the centre-post of the cottage, it's a ~7"x5" piece of timber - at least at the top.

http://i712.photobucket.com/albums/ww123/snyggapa/IMG_1639.jpg

the photo doesn't show it clearly but it finishes about 6" from the ground :roll:

 

[Matt Green]

Is this a sensible approach, and if so would 8 or so inches of exposed wall be likely enough to naturally move the damp from the wall? The void under the floor was previously filled up with the filth of the ages until it met the underside of the floorboards, so ventilation was "somewhat limited" hence I guess a damp problem! Most of our structural timber has rotted away to at least 2 feet from FFL.

That all sounds logical. There's going to be an element of suck-it-and-see with this: you have changed or repaired so many parts of the room's construction that it is difficult to relate the way the room used of function with how it will after the work. You can't really remove water from building fabric, often the best you can hope for is to move it somewhere less problematic and hopefully you've done that.

 

[Penners]

I think I would also insert air bricks in the exterior walls, within the air gap, to allow it to be cross-ventilated.

Why would you need to ventilate? There's nothing in the void between the walls that can decay (any embedded timbers excepted). Keeping the air in the void at high humidity would prevent water evaporating from the party wall preserving its surface too.

That's useful to know - thanks, Matt. I think I was guided by the mantra that I've picked up over the years: "dear air bad, moving air good"! But I take your point entirely.

 

[Penners]

i'm struggling for space so thinner is better

All of the plans that you've drawn up there look fine, Steve. But they don't address the matter of transferred noise between you and your neighbour - I understood this to one of the main planks of your problem.

The fact is that, in order to attenuate sound you need: mass, minimal bridging materials that can transmit sound and (as far as possible) zero air movement. So your only effective solution is to build a second wall of some sort, completely independent of the existing wall.

I can understand that you want to minimise your loss of floor area, but frankly by the time you've installed a studwork wall, together with the requisite amount of sound insulation material and air gap, I really don't think you'll have used much less floor space than the concrete block wall would occupy. Remember I suggested that the sound insulation was fairly optional with the concrete wall - it would improve things a bit, but the wall would be doing the the great majority of the attenuation.

If you do succeed in persuading the BCO to allow you to install a floating timber floor, you could still build the concrete wall.

 

[steveb]

yes, good point - the solution, whatever it is, will involve some kind of structure built away from the existing party wall. I have a personal preference for timber instead of blockwork but that's only based on being "traditionalist".

I guess if a leave a void between the block and the party wall that is ventilated down to the underfloor then I also need to consider heat loss through the block wall into the void as well - presumably doing the same with studwork and rockwool/celotex would be better for heat loss but worse for sound attenuation so I'll have to do some calculations and work out the best trade-off.

My remaining puzzle, assuming we can use a ventilated sub-floor, is whether adding a coat of lime render to the 8" or so of brick wall exposed in the floor void would help evaporate off any dampness faster, or just cause more to be wicked out of the ground faster ...

 

[Matt Green]

My remaining puzzle, assuming we can use a ventilated sub-floor, is whether adding a coat of lime render to the 8" or so of brick wall exposed in the floor void would help evaporate off any dampness faster, or just cause more to be wicked out of the ground faster ...

There is no way it will increase the speed of water evaporation from the wall- you are covering the wall surface. You might be doing so with something that water can move through, but its still going to restrict water movement. If the wall is not on display in the sub floor void, leave it bare.

 

[Penners]

I have a personal preference for timber instead of blockwork but that's only based on being "traditionalist".

Emotionally, I would agree with you. But a studwork wall is going to be much less massive than a block wall. And you need mass.

I guess if a leave a void between the block and the party wall that is ventilated down to the underfloor then I also need to consider heat loss through the block wall into the void as well

Yes, but Matt's opinion was that ventilation in that void would be un-necessary with two masonry structures. I wouldn't allow continuity between the sub-floor void and the void between the walls. This will allow air movement, and since you don't need this between the walls, you'll get much better insulation with still air.

So I would found the block wall in the soil, thus creating two independent voids, like this:

 

[steveb]

well, finally got planning permission approved and had the BCO round for an initial look. Looks like we got the best possible result - the BCO seems to like old buildings and accept that doing them to current building regs standards is not always appropriate - hence if we install a french drain to lower the outside ground, he's happy for us to put in a suspended timber floor throughout and leave a bare earth floor underneath.

With a bit of luck, 12 inches or so of exposed ventillated brickwork will be enough to evaporate off any drawn up water - will be speaking to the builder in the next couple of days to determine his ideas on adding soundproofing to the wall - from previous conversations he'd prefer a timber stud wall built 10mm off of the party wall , with something like rockwool flexi and a couple of layers of acoustic plasterboard to cover.

Looking at the specs it seems to be an adequate sound and heat insulation solution, so fingers crossed..