Moisture weeping into building between DPC membranes

[theoldun]

I'm still not too convinced about this being a full-on water-ingress issue, as there just doesn't seem to be any real 'flow' of water, and what little moisture there is doesn't seem to be affected by the weather at all

this is most definitely moisture that's appearing 'inside' the DPC envelope in the cavity.the room. Hence my thinking that this could be a condensation issue,

I'm at least satisfied that any moisture is going to stay localised and not creep away along the floor inside the room.

Okay, fair enough, this throws a different light, if you are convinced that it is condensation and not external water ingress, and after looking at your first sketch again, then it can only be interstitial condensation.
We only use Knauf Earthwool Dritherm full fill cavity insulation. This is guaranteed for 50 years against water ingress, but also allows water vapour to pass through. In a nut shell it is breathable if you have dot and dab board finish or light weight plaster how ever not so breathable with dense plaster.. With regard to any other full fill insulation, we have no idea if it is breathable, as we do not use any other.
Consequently approx 99.5% of the solid super structure will allow water vapour to pass through internal walls, insulation, external skin and then evaporate, how ever vapour passing through the lower 225mm of internal wall will hit the polythene cavity tray and then condense as it can not pass through the poly Visqueen barrier. The water droplets will then run down to the bottom and find their own way across bottom of tray and under first course of block work and into inside.
That is the only other possible cause.
Solution. .Areas affected, open up external brickwork, slit tray so no water can bridge over, let water run into lower cavity, make good.
Couple of other points whilst passing. In your sketch your floor DPM should be under insulation and cavity fill should have been taken down to meet floor insulation. By not doing so causes massive cold bridge.
Also it is a fact that one person can produce10 litres of water a day from normal activities.
Regards oldun .

 

[^woody^]

The builder has done half a job with the DPC

What he has done is correct, but it needed an additional tray further up

Untitled

• ^woody^
• 2 Dec 2012
• 1

 

[Isitwinteralready]

Thanks again guys for your input and advice,

Oldun, I think I should probably have shown the concrete floor insulation between the floor DPC and the concrete, so that's my fault for missing it off the original drawing rather than it being missed during the build. I was concentrating on showing the wall detail rather than the floor, so apologies.

The rock-wool you use seems to look very similar to the stuff used on my build. I'm not sure what that proves, but I thought it might be of interest.

As an aside, I had a morning outside doing some remedial work to see if I can help alleviate this issue, and I've dug away everything externally in the corners and around the DPC level, so there was some earth to move there that might have been causing issues.

I also had a poke about through the air-brick and can actually feel rock-wool on the other side of the vent-holes, which was a bit of a concern. What I'm wondering is that if this air-vent is sucking cold air into the cavity area and 'through' the cavity into the 100mm vent pipe fixed into the internal wall, and 'if' this rock-wool area is in an area open to an adjacent side of this DPC sheet 'envelope', then what would be to stop cold air condensing in this area and dropping down through the rock-wool into the bottom of the DPC envelope, and then making it's way into the building ? Is this a possibility ?

I have now plugged all the air-vent holes with pipe-lagging foam cut into square strips, and I've ally-taped over the air-vent brick as well, for a belt-and-braces aim of stopping air-flow through this air-brick, to see if the moisture issue abates over Christmas. There is still another air-brick on the opposite side of the extension, feeding air into the same under-floor area, so I don't think I'm doing anything un-towards with regards to stopping 'required' air-flow for a bit, whilst I monitor things for a time.

Obviously the sensible thing to do in terms of fixing this issue for good will be to get in around the outside DPC area and remove enough bricks to get in with a sharp knife or scissors and slit the DPC membrane so as to completely remove the DPC bridge across the cavity, and this is something I'll look to do in the longer-term.

I feel that, given the two remedial actions I took this morning with regard to clearing the corner and DPC areas and also blocking the air-vent, that I should really wait to see if either of these actions helps with the moisture issue, if only to show what might have been actually causing the moisture in the first place, if it does get better. I feel that if I got in now to cut the DPC, I'd not really be any the wiser just what's causing it to get in there, so given the relatively small amount of moisture that we're talking about, and it's localised nature, that I can afford to wait and see if I can stop the moisture first, and 'then' get in later to remove the bridge in case of any future issues. I think this would be the sensible way to proceed, but I'd appreciate your thoughts on that ?

I'm also wondering, given that a covered air-brick isn't the best thing in terms of building-health, if it might be worth thinking about removing the air-brick at some point next year and creating a 'funnel' between the cavity-side of the air-brick and the 100mm vent-pipe fixed into the internal wall. I'd hope that this would then stop any outside air getting into this particular cavity, where it's not required, and only allowing it into the house under-floor area, where it's installed to feed air to. I'd hope that this would allow the vent to be used, but remove the chance of the rock-wool becoming moist with condensation, and perhaps allowing any moisture down into this cavity. Again, your thoughts would be very much appreciated on this.

In terms of creating another DPC layer above this area, with it constructed in a manner to take moisture 'away' to the external wall, I'm a little confused as to just what this might achieve ? If the moisture was getting in from the area above, I'd agree that it would be a good idea, but it seems like a lot of building work to put something in that might not particularly solve my issue, as we're still not really sure where the moisture is coming from, with the only thing we do know being that it's appearing at the bottom of the DPC 'trough' and making it's way inside the building. If the issue is lower down, where we think it might be at the moment, then wouldn't this upper DPC be a little redundant, and still leave this particular issue if we can't find a way to remedy it ?

Thanks again for all the input and time taken for the replies. This seems like a long post, but I do want to understand the issues here, and I'm trying to go into detail where I'm not 100% clear on just what we're talking about.

Cheers,

Isitwinteralready

 

[^woody^]

In that situation depicted in the sketch, the chances of condensation forming in the cavity are minimal

Whereas the chances of moisture running down the inside face of the outer skin are 1000's of times greater

 

[Isitwinteralready]

Hi Woody,

Whereas the chances of moisture running down the inside face of the outer skin are 1000's of times greater

I do appreciate that, but water running down the inside face of the outside wall isn't the issue here so much as water getting on the 'other side' of a DPC membrane running down that same wall.

I think it's right to make that distinction here, given that the builder has made what he described as a DPC 'envelope' inside the cavity. The shame being that this 'envelope' is delivering moisture to the wrong side of the cavity, unfortunately...

I should point out that the area above the ground line externally is rendered with skim and pebble-dash, so I think that should help alleviate any risk of actual water ingress through the wall above that level, would you agree ?

Does this make sense, or am I wide of the mark here ? I think for moisture to be running down the inside of the cavity from above, and dropping into the envelope from there, it'd have to be getting through pebbledash, skim, and then the brick itself, which seems unlikely does it not ? This is the main reason I've been concentrating my thoughts on the lower area, and condensation as being a suspect.

Thanks again for your input, it's much appreciated. I think it's very helpful to go through the possibilities, and certainly writing about the issues and possible solutions here has certainly helped me a great deal in getting my head round just where we are and what might be done to help, so it really is helpful.

Cheers,

Isitwinteralready

 

[Isitwinteralready]

Hi,

Just wanted to come back to this thread to update everyone.

I blocked the outside vent holes with strips of foam and taped over it as well for good measure, and 10 days later the moisture coming into the room has stopped completely. This is after a good few months of a small, constant stream of moisture coming in between the two damp proof layers.

Whilst inserting the foam strips I noticed that there was cavity insulation directly behind the vent brick. This means that any air sucked into the vent is having to pass through this insulation before getting into the 100mm pipe in the internal wall, before it goes through to my under-floor space under the house.

It seems to me that the most likely thing happening here is that the moist air is leaving moisture in the cavity insulation, where it's eventually dropping into the damp-proof 'envelope', depositing at the bottom where it's got nowhere else to go. Eventually this is pooling enough to make it drain into the room between the two damp-proof membranes internally.

Whilst I'm not too happy that this is happening, at least I seem to have found the cause of the issue, given that it's stopped so suddenly after blocking the vent.

I think I've got two things to sort out here, but I'd welcome any feedback as to what I think is a way ahead.

I think I need to remove the vent brick, remove the cavity-insulation in the vicinity of the vent-brick, and perhaps make some sort of cowl that will sit in the cavity directing air from the cavity-side of the air-brick into the cavity side of the 100mm round pipe in the internal wall. My thinking here is that I need to ensure air entering the air brick is only going where it's supposed to go, which is through the 100mm pipe internally, through to my house under-floor space.

The second thing I think I need to do, given that this incident has proved that any water getting into my cavity in this area is going to make it's way into my extension, is to cut the bottom of the damp-proof 'envelope' that is currently sitting there waiting to catch any water and deliver it into my room. I think the removal of two or three bricks externally and a slim arm in to cut along the cavity, to open up with bottom of the damp proof envelope will ensure that any more moisture ever getting into that area will simply drop into the lower cavity area and not be an issue.

Given that I've got a window in the wall above the air-vent, and then a velux in the roof directly over this area, I need to make sure that any future issues with water that might occur due to those to fittings will not give me large amounts of water inside my room. I guess I can think myself lucky in a way that I've discovered this issue with a relatively small amount of moisture, rather than finding out there's an issue after a roof or window leak.

Anyhow, I just wanted to come back and update the thread. If anyone could give a quick sanity check on my ideas to improve things, and close this issue out for good, then I'd be very grateful for any further feedback.

Thanks again for all the interest and advice up to this point, I realise these are quite long posts, but I want to include details that might be relevant, knowing all too well that it's sometimes the small things that don't get mentioned that cause the real problems...

Cheers,

Isitwinteralready